Imagine that you have a big coloring book with lots of pages. Now, each page is a rectangle shape, right? Well, some rectangles are wider than others. Just like some picture frames are wider than others too!
When we say "widescreen" we usually mean a certain shape of picture that's wider than it is tall. It's like a rectangle that's been squished so it's wider.
This is different from a "regular" picture shape which is more like a square or a rectangle that's taller than it is wide. TV shows and movies can come in both shapes, and some movies are filmed and shown in widescreen because it gives you more of the big picture.
But it's important to remember that no matter what shape the picture is, it's still a way to enjoy a cool story or see some fun characters!