When something fossilizes, it becomes a fossil, meaning it leaves an impression in the Earth that far outlives the organism.
Fossils are remnants left in rock of a living creature: the remnants have been petrified over many years and they leave an impression of what the animal was like. Fossilizing is a word for this process, which happens slowly over time. If dinosaurs had not fossilized, we wouldn't know much about what dinosaurs were like. The most likely parts of a creature to fossilize are hard parts like bones and shells.