What does heap mean?

This means that sysprof believes a function was called from somewhere in the program's heap (where malloc allocates memory) rather than the code section (where executable code normally lives.) There are several possible explanations.

Languages like Java can generate executable code while a program is running and store it in the heap. Sysprof is accurately reporting the situation in this case.

C and C++ compilers can optimize away information needed to determine a function's caller, so it is mistaken for [heap]. You can still tell how often each function is called by the program, but not always from where.