Rec. 601 is like a recipe that tells people how to make things look good on TVs and computer screens. It tells TV and computer makers how they should mix colors to make sure that everything looks the same from screen to screen.
It's kind of like when you follow a recipe to make cookies. The recipe tells you how much flour, sugar, and butter you should put in so that the cookies turn out just right. Rec. 601 does the same thing, but instead of ingredients for cookies, it tells TV and computer makers how to mix colors to make sure everything looks the same.
For example, if you're watching a movie on your TV and the colors look really weird, it might be because the TV doesn't follow the rec. 601 "recipe" correctly. And if that happens, you won't be able to see the movie the way the people who made it intended. So rec. 601 is very important for making sure that we can all watch and enjoy TV and movies the way they were meant to be seen.