Extinction happens when there are no remaining individuals of a species still alive. Once a species is extinct it can never come back.
Several factors can contribute to the extinction of a species. The environment may change over geological timescales, for example when the climate becomes much colder or hotter. A new predator may arrive and hunt the species faster than it can reproduce. A new disease may spread through the population. A new competitor may arrive and win in the competition for food, water or space. Catastrophic events such as a massive volcanic eruption or an asteroid collision can also wipe out species.
Human activity is another major factor. People can destroy habitats, hunt animals, and pollute the environment, all of which reduce the number of individuals in a species. If the population falls far enough, the species may disappear completely.
A species is more likely to survive if it already shows variation, because some individuals may have characteristics that let them survive a change. If no individuals can cope with the change, the species becomes extinct.