What Are The "Machines" On An Acoustic Guitar?

The "machines" are the tuning devices that hold the strings in place on an acoustic or classical guitar, and tighten the strings on the head of the guitar.

There are six strings on an acoustic or classical guitar,
E G B D A E.   There are therefore six tuning machines and six pegs that fit into them through the holes that are bored in the head of the guitar.

The tuning machines have a gear that allows you to tighten the string by slowly turning the gear that twists the peg in its place and causes the string to wrap around it.  The string fits through a hole in the peg, and then wraps around itself as much as you twist the gear.

These devices are delicate, and they can break if the guitar is laid down on its head and not set on a stand when it is not being used.

There are some tuning devices that are lockable.  You can lock the machine in place at a certain level of tightness so that bumping it will not cause the string to change its pitch.