Meaning of "~" (tilde) symbol in C++?

It is the destructor. It gets called when you destroy (reaching end of scope, or calling delete to a pointer to) the instance of the object.

In the context you're using it, it defines a destructor.

In other context such as the following one, it's also called bitwise negation (complement):

int a = ~100;
int b = ~a;

Output: (ideone)

-101
100