The guitar is a musical instrument. Guitars consist firstly of a hollow body of wood, the so-called body, secondly of the neck with the fingerboard and thirdly of the head with the pegs. Strings are stretched from the head to the bridge on the body. When you pluck a string, it begins to vibrate and produces a sound. This tone passes through the bridge into the hollow body, which amplifies the sound.
Most guitars have six strings. They are of different thicknesses. The thickest one sounds the deepest. The thinner the strings and the tighter they are stretched, the higher they sound. But there is a third way to change the pitch:
On the neck is the fingerboard. There are frets on it. These are slight elevations. Between them are the so-called frets. If you press a string with your finger in a certain fret, you change the pitch: the closer to the body you press the string in the fret, the shorter the string becomes and the higher the sound. This is the same as with stringed instruments, except that there are no frets.
At the top of the fingerboard is the head with the tuning pegs. These are used to tension the strings so that they produce the right tone. This is called tuning the strings. The wood and the strings warp with the heat or humidity of the air.