Okay so imagine you're reading a book, but instead of just looking at the words in a straight line, your eyes need to jump around to different parts of the page to understand what's going on. That's kind of what happens when someone reads music, too.
When you look at sheet music, there are a bunch of symbols and notes on a page that tell you how to play a song. But since there's usually a lot going on at once, your eyes have to move around the page to make sense of it all.
So, you might start by looking at the beginning of a song, where there's usually a big letter that tells you what key you're playing in. Then you might look at the first few notes to figure out what the melody sounds like. But then your eyes need to jump down to the next line to see what other instruments are doing, or what chords to play on a guitar, for example.
It's kind of like playing a game of "eye spy" where you have to find different things on a page in a certain order to understand the whole picture. And just like with reading a book, the more practice you get doing it, the easier it becomes!