Seating capacity is like how many chairs you have for your friends to sit on. Imagine that you're having a party, and you have 10 friends coming over. You need 10 chairs so that everyone can have a place to sit.
Seating capacity is the same thing, but for bigger places, like a movie theater or a stadium. A movie theater, for example, needs a certain amount of seats so that everyone can watch the movie comfortably. If there aren't enough seats, some people might have to stand or sit on the floor, which isn't very fun.
The seating capacity is the maximum amount of people that can comfortably sit in the seats provided in a certain space. It's important to make sure that there are enough seats for everyone, so no one has to stand or sit on the floor, and everyone can enjoy the event or activity that they came to see.