People usually bite their nails when they are strained or nervous.
We use the expression nail-biter to describe things that include anxiety or tension, things that make us in a state and it doesn’t mean that you need to bite your nails in reality. It’s just a figurative way to express tension and stress.
Let’s see some examples :
– I can’t wait till the game ends. This match is a realnail-biter.
– all events were suspenseful and the story was mysterious. Let me say : It’s a nail-biter movie and you should watch it !