APA style is like a special set of rules for how to write papers and essays. It's kind of like when you play a game and there are rules you have to follow to play the game correctly.
So, when you write using APA style, you have to use a certain format for things like your font (the way your words look on the page), your margins (how much space you leave around the edge of the page), and how you organize your paper (like having a title page and different sections with different headings).
You also have to use a special way of citing (giving credit) to other people's work that you use in your paper. This means you have to write where you got your information from and who said it so that people know it's not your own idea.
Think of it like this: when you write a paper using APA style, you're following a set of rules to make sure your work is clear and easy to understand, and also to give credit to other people's work that you use in your paper.