What is a fret on a guitar?

Simply, a fret on a guitar is one of the many metal bumps that you'll find just below the strings on the neck of the guitar, called the fretboard:

Most guitars have at least 15 frets, but there are some guitars that have 24 or more. These are great guitars if you're interested in playing shred metal or crazy lead guitar stuff.

Most acoustic guitars have at least 15 frets, and most electric guitars have at least 19. 

Photo Cred: Tango by Louise64, CC0 Public Domain