A film series is like a bunch of movies that are connected to each other. It's like watching different parts of a really long story.
Imagine you are playing with your toys and you have a bunch of different toys. You can make up stories for each toy, but sometimes you want to connect them all together and make one big story.
Or maybe you like books and you've read a series before. It's like that, but instead of words on a page, it's a bunch of movies that you watch on the TV or at the movie theater.
The movies usually have the same characters in them and they follow a story line. So, it's kind of like watching a really long TV show, but instead of episodes, it's different movies.
Sometimes, the movies can be watched in any order, but other times, you have to watch them in a certain order to understand what's going on. It's like putting a puzzle together, each piece is a movie and when you put them all together, you get the whole picture.
Examples of film series are Harry Potter, The Lord of the Rings, and Marvel Cinematic Universe.