Fiber is material your body can’t digest. It’s often found in plants, so fruits and vegetables are high in fiber. This (along with their high water content) is one of the main reasons that a diet of entirely fruits and vegetables will have very few calories. A cow could digest that lettuce, but our stomach doesn’t really know what to do with it. There are also lots of plants that other animals eat (like grass and wood) that have so little nutritional value to us that we don’t even bother trying.
It’s unclear what you mean by “bad effect” (please, if you take nothing else from this, understand that associating vaguely negative feelings and concepts with huge food groups is not mentally or physically healthy behavior), but it is possible to eat too much fiber. It may fill you up before you have eaten enough calories to sustain yourself. Some people also get gas from certain kinds of fiber.
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