The stack is where recursive routines store their active variables when recursed, and each recursion pushes another set of data (stack gets deeper) until things unwind and get taken off.

You have finite space on the stack; if rountines nest too deeply, you run out.

It's what happens if you stack work in an inbox as received and work only on the top item. If you get too far behind, the stack falls over.

The way to fix it is to redesign your code.