Fretting is a fancy word that means moving your fingers back and forth on a guitar or other stringed instrument to make music. When you push down on a string in a certain place, it makes a different sound than if you were to push down on the same string in a different place. That's because the string is shorter in some places than in others.
The places where you press down on the string are called "frets." On a guitar, there are usually around 22 frets. Each one is a little metal strip that separates the string into different lengths. Each time you press down on a fret, you're shortening the length of the string and therefore changing the note or tone of the sound it makes.
Fretting is a really important skill for guitar players because it lets you play lots of different songs and sounds. It takes practice to get good at it, but it's worth it if you love making music!