An extinction happens when there are no remaining individuals of a species still alive. Once a species is extinct it cannot return.
Several factors can contribute to the extinction of a species. A new predator may arrive that the species has no defence against. A new disease may spread through the population. A new competitor may take the food or other resources the species needs, so that the species cannot get enough to survive.
The environment can also change over geological time. Changes in climate can mean that a species no longer has the conditions it is adapted to. Extinction can also follow a single catastrophic event, such as a volcanic eruption or an asteroid collision, which changes conditions across the whole planet very quickly.
If a species cannot adapt, or reproduce fast enough, to cope with these changes, all of its members die and the species becomes extinct.