Computer definition: In the context of computer science

In the context of computer science, we aren't interested in stating a fact, but rather, in testing a true/false condition for the purpose of deciding whether to do something. A space on your computer where extra downloaded video data is temporarily stored is the buffer. A video progress bar is probably the best visual example of this: That video is 5:05. Its total play time is represented by the white portion of the bar (which would be solid white if you had not started watching it yet.) I have been trying to find a good definition , and get an understanding, of what a thread really is. It seems that I must be missing something obvious, but every time I read about what a thread is,... How does a stack overflow occur and what are the ways to make sure it doesn't happen, or ways to prevent one?