"Fiber" is something that's contained in certain foods. It's a part of a plant that your body can't digest. Some foods that contain fiber include:
Fiber is supposed to be good for you because it keeps your digestion regular. In other words, when you eat plenty of fiber, you use the bathroom at normal times each day.
When you're talking about food, you can say that something "has fiber":
Beans have a lot of fiber.