That's a complicated question that ecologists and other biologists are still trying to understand. Things like bacteria can often adapt fast because they can quickly change - it only takes 20 minutes for one bacterium to divide into 2 bacteria, and there are millions of them, so lots of them have some gene that has changed, and maybe one of those has a changed gene that helps it adapt to something like growing in the presence of an antibiotic. So that one 'antibiotic resistant' bacterium can grow and divide and adapt to the antibiotic.

Adapting basically means being able to change. People are adaptable, too, because we are intelligent enough that we can figure out ways to change when we need to. We invent things to help us, and we study ways to help people get along with each other, and lots of other things.

Some things are probably so tough that they aren't harmed by much of anything, like cockroaches. Other things are more needy and delicate, like hummingbirds that need so much food to keep their wings beating fast. But hummingbirds have adapted to us by learning to eat from our hummingbird feeders.