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Showing content from https://en.cppreference.com/w/cpp/language/../error/../language/cv.html below:

cv (const and volatile) type qualifiers

Appear in any type specifier, including decl-specifier-seq of declaration grammar, to specify constness or volatility of the object being declared or of the type being named.

[edit] Explanation

Any (possibly incomplete) type other than function type or reference type is a type in a group of the following four distinct but related types:

These four types in the same group have the same representation and alignment requirements.

Array types are considered to have the same cv-qualification as their element types.

[edit] const and volatile objects

When an object is first created, the cv-qualifiers used (which could be part of decl-specifier-seq or part of a declarator in a declaration, or part of type-id in a new-expression) determine the constness or volatility of the object, as follows:

Such object cannot be modified: attempt to do so directly is a compile-time error, and attempt to do so indirectly (e.g., by modifying the const object through a reference or pointer to non-const type) results in undefined behavior.
Every access (read or write operation, member function call, etc.) made through a glvalue expression of volatile-qualified type is treated as a visible side-effect for the purposes of optimization (that is, within a single thread of execution, volatile accesses cannot be optimized out or reordered with another visible side effect that is sequenced-before or sequenced-after the volatile access. This makes volatile objects suitable for communication with a signal handler, but not with another thread of execution, see std::memory_order). Any attempt to access a volatile object through a glvalue of non-volatile type (e.g. through a reference or pointer to non-volatile type) results in undefined behavior.
Behaves as both a const object and as a volatile object.

Each cv-qualifier (const and volatile) can appear at most once in any cv-qualifier sequence. For example, const const and volatile const volatile are not valid cv-qualifier sequences.

[edit] mutable specifier

May appear in the declaration of a non-static class members of non-reference non-const type:

class X
{
    mutable const int* p; // OK
    mutable int* const q; // ill-formed
    mutable int&       r; // ill-formed
};

mutable is used to specify that the member does not affect the externally visible state of the class (as often used for mutexes, memo caches, lazy evaluation, and access instrumentation).

[edit] Conversions

There is partial ordering of cv-qualifiers by the order of increasing restrictions. The type can be said more or less cv-qualified than:

References and pointers to cv-qualified types can be implicitly converted to references and pointers to more cv-qualified types, see qualification conversions for details.

To convert a reference or a pointer to a cv-qualified type to a reference or pointer to a less cv-qualified type, const_cast must be used.

[edit] Notes

The const qualifier used on a declaration of a non-local non-volatile non-template(since C++14)non-inline(since C++17) variable that is not declared extern gives it internal linkage. This is different from C where const file scope variables have external linkage.

The C++ language grammar treats mutable as a storage-class-specifier, rather than a type qualifier, but it does not affect storage class or linkage.

[edit] Keywords

const, volatile, mutable

[edit] Example 
#include <cstdlib>
 
int main()
{
    int n1 = 0;          // non-const object
    const int n2 = 0;    // const object
    int const n3 = 0;    // const object (same as n2)
    volatile int n4 = 0; // volatile object
 
    const struct
    {
        int n1;
        mutable int n2;
    } x = {0, 0};        // const object with mutable member
 
    n1 = 1;   // OK: modifiable object
//  n2 = 2;   // error: non-modifiable object
    n4 = 3;   // OK: treated as a side-effect
//  x.n1 = 4; // error: member of a const object is const
    x.n2 = 4; // OK: mutable member of a const object isn't const
 
    const int& r1 = n1; // reference to const bound to non-const object
//  r1 = 2; // error: attempt to modify through reference to const
    const_cast<int&>(r1) = 2; // OK: modifies non-const object n1
 
    const int& r2 = n2; // reference to const bound to const object
//  r2 = 2; // error: attempt to modify through reference to const
//  const_cast<int&>(r2) = 2; // undefined behavior: attempt to modify const object n2
 
    [](...){}(n3, n4, x, r2); // see also: [[maybe_unused]]
 
    std::system("g++ -O3 -Wa,-adhln ./main.cpp"); // may issue asm on POSIX systems
}

Possible output:

# typical machine code produced on an x86_64 platform
# (only the code that contributes to observable side-effects is emitted)
main:
    movl    $0, -4(%rsp) # volatile int n4 = 0;
    movl    $3, -4(%rsp) # n4 = 3;
    xorl    %eax, %eax   # return 0 (implicit)
    ret
[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior CWG 1428 C++98 the definition of 'const object' was based on declaration based on object type CWG 1528 C++98 there was no requirement on the number of occurrences
of each cv-qualifier in the same cv-qualifier sequence at most once for
each cv-qualifier CWG 1799 C++98 mutable could be applied to data members not declared
const, but the members' types may still be const-qualified cannot apply mutable to data
members of const-qualified types [edit] See also

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