A need is something essential: people need food, water, and shelter. When you lack them, you're in need.
Besides physical needs like sleep, people have lots of psychological needs: like for love and friendship. To pass an exam, you need to study. To lose weight, you need to exercise more and eat less. Drug addicts feel a physical need for their drug — so do smokers. Often, we feel we need things that we actually just want: like a new video game or car. When someone is trying hard to get something, they might say "I need it!"