Okay kiddo, so you know when Mommy and Daddy dress up really nice to go to a fancy party or wedding? Well, sometimes they don't have to wear their fanciest clothes, but they still have to look nice. That's when they wear something called "semi-formal wear."
Semi-formal wear is like a middle ground between super fancy clothes (like a ball gown or a tuxedo) and everyday casual clothes (like jeans and a t-shirt). When people wear semi-formal clothes, they still look dressed up, but not too dressed up.
For boys, semi-formal wear usually means a dress shirt, dress pants, and a tie. Sometimes they also wear a jacket called a "blazer." For girls, semi-formal wear can be a dress that's a little less fancy than a ball gown, or dressy pants and a nice blouse.
People wear semi-formal clothes to all kinds of events, like a school dance, a fancy dinner party, or even a bat mitzvah or bar mitzvah. The important thing is to look nice, but not too fancy, and feel comfortable in what you're wearing.
So there you have it, kid - semi-formal wear is fancy-ish clothes that are perfect for looking put-together without being too dressed up.