What does nervous mean?

Nervousness is a feeling of uneasiness, anxiety, uncertainty or worry. Signs of nervousness might include: pacing, jitters, twitching, excess movement or frequent shifting, sweating, accelerated heart rate, racing thoughts, stuttering, mumbling, and more.

Additionally, the nervous system, as mentioned in the initial answer, is the system of your body that processes information to your brain from other parts of your body. If you place your hand on a hot stove, your nervous system is what carries the "pain signal" to your brain and causes you retract your hand.