OK, so get some really good dessert that your entire family really likes. Maybe some kind of pie or something. Now, you take a piece, go away and come back. But act as if you expect there to be only one piece gone, and you want to do some calculation that you expect to use a pie with just one piece gone. Now you and another family member try to cut a piece at the exact same time, even cutting in the exact same area, so that it is overlapping.
That is the basic feel of multithreading. Well, sort of. I was just really hungry for some pie.