The purpose of a metronome is to enable you to stabilize the pace at which you play a piece of music.
Metronomes are available in the traditional wooden wind-up style, as battery-powered electronic devices and through the internet. They all function similarly.
While they are a time tested and proven tool for steadying a tempo, they can prove to be notoriously frustrating when initially included in your playing.
So, it is good news that finally, there is a clearly written, easy to follow, effective method of learning to do just that, include a metronome in your music practice.