In c++ what does a tilde "~" before a function name signify?

It's the destructor, it destroys the instance, frees up memory, etc. etc.

Here's a description from ibm.com:

Destructors are usually used to deallocate memory and do other cleanup for a class object and its class members when the object is destroyed. A destructor is called for a class object when that object passes out of scope or is explicitly deleted.

See https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_74/rzarg/cplr380.htm

As others have noted, in the instance you are asking about it is the destructor for class Stack.

But taking your question exactly as it appears in the title:

In c++ what does a tilde “~” before a function name signify?

there is another situation. In any context except immediately before the name of a class (which is the destructor context), ~ is the one's complement (or bitwise not) operator. To be sure it does not come up very often, but you can imagine a case like

if (~getMask()) { ...

which looks similar, but has a very different meaning.

It's a destructor. The function is guaranteed to be called when the object goes out of scope.