I bought this Metronome a little while back and I have no ****ing clue how to use it. I go on a lot of Forums about learning the guitar and it always says use a Metronome. I know how to turn it on, but then there is a button for Beat and it goes 0,2,3,4,5,6, also the Metronome goes from 40-208. In case you needed to know that.
The beat is probably which beat gets emphasized. It's useful if you're playing in different time signatures. 4/4 is most common so I'd recommend keeping on 4/4 or 0 for now. The 40-208 is the BPM (Beats Per Minute). That determines the tempo of what you're playing. Usually you should start slow (60 BPM or so) and work the BPM up in 10 BPM increments.

Hope that helps...