Okay kiddo, have you ever played a game where you have to guess what's missing from a picture or a list of things? That's kind of what purposeful omission means. It's when someone leaves something out on purpose to make people guess or think harder about what's not there.
For example, imagine I have a bag of candy and I tell you there are five different flavors, but I don't tell you what they are. That's a purposeful omission because I left out important information that would help you know everything about the candy. I'm doing this on purpose to make you curious or maybe to make you want to try the candy and figure out the flavors yourself.
Sometimes people use purposeful omission in stories or movies to make them more interesting or mysterious. They might leave out important details until the end of the story or they might only hint at something important to make you guess what's really happening.
So basically, purposeful omission is like when you play a guessing game by leaving out information on purpose or when you tell a story and only give some of the details to make it more fun or interesting.