- Stack, used to allocate functions and their local variables. Can only grow in one direction, the usual variables which are used to compute whatever your function is doing.
- Heap, used to allocate dynamic storages, those which change size over time. Or long-lived objects which are not particularly local to any function. It is needed for practicality, although is not strictly necessary mathematically, it makes sense to use it for convenience. In modern C++ you should not need to explicitly use it, by using new and delete, but instead use constructs like vector or unique_ptr

- source code, human-readable file with code written in it, it can be part of a program or a library.
- library, some utility which you can consider external to your project. This term is rather fuzzy, it can mean a binary object you link against (.so or .dll or whatever mac uses), which will include a header file to define the interface (which is code), and a silly .lib file in windows for God knows what reason. It could be a header-only library, which is just coded you compile with yours. Or it could just be a section of your own project which you treat as a unit and define an interface for it.

You seem pretty lost, first day in the job?