A metronome is a time keeping device that musicians use to assist in maintaining tempo.
Metronomes can be heard audibly with a clicking sound, or can display the tempo visually such as with flashing LED lights.
The tempo is set on a metronome to the desired BPM (Beats Per Minute) and the musician plays along with it to ensure that they are staying in tempo.
Metronomes can be used as a practice tool but can also be used in other applications. In the recording studio, drummers will often play along to a "click track" which helps them maintain a consistent tempo throughout the song. 
Metronomes come in all shapes and sizes and can be found as battery operated digital devices, electronic units, or manually operated "wind up" devices.
When learning a new chord progression, lead line, guitar riff or scale; it is often difficult to play the piece at the desired speed the first time.
When it comes to guitar, the best way to learn to play fast is to play slow! This is where the metronome comes in. By setting the BPM to a slow tempo with a new progression, riff or scale; you can work out the passage at a slower pace. 
Then by gradually increasing the BPM's and playing along, you will learn how to play the passage faster until you are finally able to play it at the desired speed, and, in tempo.
For example. A new progression or riff you are learning may run at a normal tempo of 120 BPM but you can't play it at that speed to begin with. You might try setting the metronome to half the normal speed (60 BPM) and learn to play it proficiently at that speed. Then you could increase to 80 BPM until you can play it smoothly at that speed.
By repeating this process, gradually increasing the BPM's at each stage, you will eventually be able to play it at full tempo with no problems because now your fingers have learned to play the piece through repetition and you no longer have to "think" about it.
It cannot be overstated what an important tool a metronome is for a beginning guitarist. They can be purchased fairly inexpensively and are well worth the small investment.
We would highly recommend that you pick one up and see what it can do to help improve your playing.
Please do not hesitate to contact us with any questions.