It's an area in storage that contains pointers to other memory addresses, sub-routine execution states, or any number of other values.
I haven't seen a stack overflow in years, so this must be an older program. If it occurs under emulation, like Virtual PC running DOS, you can increase the number of stacks allocated in CONFIG.SYS. The format of the parameter varies based on the version you're running.
What are you running when you see it?