What's a buffer overrun? Well, a buffer is a piece of RAM. A chunk of memory. Programs store data in them.

Sometimes, due to bugs, programs put data not only in a buffer where it belongs, but in other RAM that's next to the buffer. This screws up whatever information was in that other RAM.

The message you saw is displayed by error-checking code in the Visual C++ library which is produced by Microsoft. Microsoft is a big company so it's not appropriate for them to tell you "the program's info in RAM got screwed up." Instead they have to sound professional and tell you that the program's internal state got corrupted. It means the same thing.