ViewVC is a computer program that helps people look at different versions of code, kind of like a time machine for computer programs! So if you've ever made a drawing and then made a few changes, you can think of each version of the drawing as a different "code."
ViewVC helps you see the "history" of all those different versions, so you can see when changes were made and what was changed. And since sometimes people make different versions of a code at the same time, ViewVC helps keep all those versions organized so you don't get confused.
Think of it this way: If you were playing with a bunch of different colored blocks and you wanted to see how your tower of blocks looked at different times, you could use ViewVC to take a picture of your tower each time you made a change, so you could look back and see how it grew and changed over time.