A Metronome is nothing but simply a device ticking at selected rate to mark musical time. To be more precise, it beeps or clicks at a steady rate, offering you a firm tempo to sing or play along with.
A song normally sticks to a steady tempo (the speed of a song) or else it will sound dragged, rushed or disorderly. We humans are always prone to fall behind or speed up while singing or playing. A metronome helps to improve your timing or sense of rhythm as it is accurate and consistent.
Before using a metronome you need to set it to your desired tempo. Usually it is set by deciding the number of beats per minute which is called BPM (beats per minute). When you count how many beats are there in one minute of a song played at a particular tempo, you can easily find the BPM of that song. 60 BPM is a bit slow as it only covers one beat per second whereas 240 BPM is quite fast (4 beats per second).
For a beginner, it is always advisable to begin in a slow tempo and gradually you may increase the tempo. In this way you can improve your timing and skill. Practicing only with a metronome may be a bit tedious but it is important even if you are a beginner or professional.