* The fret on the guitar was loose. (The metal strip on the neck of a guitar)
* He had a lot of frets about the exam. (Worries or anxieties)
* Don't fret, everything will be alright. (A mild expression of worry or anxiety)
It can also be a verb, meaning to worry or be anxious:
* Don't fret about it, I'll take care of it.
* She fretted over her children's safety.