What does it mean if some guitar frets allow the to make sound?

Frets that allow the string to make a sound are called "fretted" frets. They are made of metal and are slightly raised from the neck of the guitar. When you press down on a fretted fret, the string is shortened, which changes the pitch of the sound.

In contrast, "unfretted" frets are made of wood and are not raised from the neck of the guitar. When you press down on an unfretted fret, the string is not shortened, so the pitch of the sound does not change.

Some guitars have both fretted and unfretted frets. This allows the guitarist to play a wider variety of notes and chords.