Okay kiddo, have you ever played outside on the grass? Grass courts are like big grassy fields but they are specifically made for playing tennis on.
The grass is cut really short and is super flat - this makes the ball bounce lower and move faster, which means players need to run around quickly and be extra careful not to slip.
Also, grass courts can be a bit harder to predict where the ball will go because the grass isn't completely even - some parts may be faster than others. So players really have to pay attention and adjust their shots based on the way the ball bounces.
Some really famous tennis tournaments are played on grass courts, like Wimbledon. People say it's a really tough surface to play on but it's also really exciting to watch!