What is extinction and why does it occur?

the termination of an organism or of a group of organisms, usually a species. it occurs when the last individual of a plant or animal species dies out or due to human activity, such as over hunting or through habitat destruction.

Explanation:

this should break it down clear enough

Extinction is the dying out of an entire species. Extinctions have occurred regularly throughout Earth's history. At certain times, major climate changes, catastrophic events such as large meteorite impacts, or massive volcanic activity on earth have caused the Extinction of large numbers of species.