This free online metronome plays a click at the tempo, in beats per minute (bpm), which is specified in the input field.
Metronomes are useful when practicing an instrument, or when working on your sense of rhythm. It is a helpful tool for both beginners, and for more experienced musicians alike.
A common way to use a metronome when practicing a difficult passage is to reduce the tempo to a point where you are able to play the notes without making an errors at all - even if this is extremely slowly. Then very gradually increase the tempo, over the course of several days or weeks until you are able to play at the desired speed.