Extinction is what happens whenever a species is dead and gone and is never coming back.
It is what happens to the vast majority of life. As far as research can currently tell, 99.999-whatever percent of all things that have ever been alive on Earth have gone extinct.
It is essential and normal and natural for natural selection, for evolution to occur, that some species go.
Natural selection is a theory of evolution came up by Charles Darwin, and Alfred Russel Wallace and various other people kind of contributed to it. It’s the idea that in order for you to keep going you have to be better than whatever’s trying to eat you, whatever you’re trying to eat and your fellows who are also performing the same kind of job.