IPhone help: what does this mean? S?

I checked the "Usage" on my iPhone and it says 5.9 GB Available and then beside it says 7.6 GB Used. How did this happen? How can I fix it? What does it mean? Please help!

Added (1). 7.6 GB is more than 5.9 GB, that's why I'm freaking out.

It says that you have 5.9 GB available out of all the storage space on your phone for apps, video, and music. You've filled up 7.6 GB already.

They is no need to panic. Nothing's wrong.