What Is Turbulence?

I don't have a deep fear of flying, but i'm a little bit afraid. Usually turbulence is very mild and it's nothing to worry about. But sometimes turbulence can get pretty heavy, ad it can seem a lot scarier. I've only had one experience like this earlier this year. It was the worst turbulence that i've ever experienced. People were screaming and crying, and it was really intense. Luckily it didn't last for too long, and everything went back to normal. Everyone on the airplane was just waiting to be on land again.