Here's a quick introduction to one way you can use I IV V, although there is a TON more to say on this topic. The guitar is tuned to 4ths, except for the G to B string, which is a major third. That means that every time you switch strings, staying on the same fret, you're moving up (or down) a fourth.
For example, on 6th string, 5th fret, you have an A. Move up one string to the 5th string, and you have the D. Now, move up two frets, and you find the E, and there's your I IV V in the key of A, the three most important chords in that key. This works in both major and minor keys.
Learning a bit about I IV V can really help you reduce how much you need to memorize your fretboard, because you'll already know the relationships of the notes nearby. Cool stuff!