Nerve has several meanings:

1. Physical courage or boldness, especially when facing something difficult or scary. When a diver climbs to the high platform for the first time, she needs nerve to actually jump. When you raise your hand to answer a tough question in class, that takes nerve too. People might say “she's got nerve” admiringly about someone willing to try something risky, or “he lost his nerve” about someone who backed down at the last moment.
2. The actual physical structures in your body that carry signals between your brain and the rest of you. Nerves are like biological wires: they tell your muscles to move, send information about pain or temperature to your brain, and help coordinate everything your body does. When you touch something hot, nerves instantly send that warning to your brain. When your foot “falls asleep,” it's because pressure temporarily interferes with the nerves there.
3. Rude boldness or audacity, usually used negatively. If someone cuts in line at the water fountain, a student might say “What nerve!” or “She's got some nerve!” This meaning suggests the person is being inappropriately bold or disrespectful. It's the difference between brave confidence and pushy rudeness.

The phrase get on someone's nerves means to annoy them persistently, like a younger sibling who keeps poking you while you're trying to read.