Okay kiddo, let's say you have a really big puzzle that has a lot of pieces. But you have a lot of friends who like puzzles too! So you all decide to work on the same puzzle, but each one of you takes only a few pieces at a time.
That's kind of like data parallelism. Instead of working with one big piece of data all at once, we break it up into smaller pieces and have a bunch of computers or processors work on it at the same time. This is especially useful when you have a LOT of data to work with, because it can save a lot of time.
Each computer or processor works on its own little piece of the data, putting it together like a puzzle. When they're all done, the pieces are joined together to form the complete data set. This is called parallel processing, and it's a really cool way to get things done faster!