Imagine you are watching a show on TV. The picture you see is actually made up of tiny dots called pixels. The more pixels there are, the clearer the picture is.
Back in the day, TVs used to have 405 lines of pixels up and down the screen to create a picture. This was a really old and basic kind of TV system.
Let's pretend you have a coloring book with 405 pages. Each page has a little dot on it, and each dot is a different color. When you flip through the book, really fast, the dots blend together to make a picture. That's kinda how the 405-line TV worked.
The picture wasn't super clear, but people still enjoyed watching it. Nowadays, TVs have a lot more lines of pixels, making the picture much clearer and easier to see.