It works on the principle of numbering the chords in a key from one to seven, according to their place in the scale.  Roman numerals are often used with large numerals for major chords and small ones for minor.  Not everyone does this, nor do they always use Roman numerals, it's up to you.

So, for the key of D:

Chord charts written out using this system can be easily transposed on the fly, if you need to play in different key - to suit a different singer, for example.  If we take our example from the previous page, Burning Love, it is in the key of D, so the I chord is D, the IV, G, the V, A and the vi, Bm.  If we wanted to play it in the key of G it is easy to work out that the chords would be I – G, IV – C, V – D and vi  - Em.  Once familiar with the chords which go with the different keys, you can easily play any song in any key.  Incidentally, the I, IV, V vi chord sequence used in this song is very common.  Once you're familiar with it, you'll hear it everywhere!