Up to this point you have learned seven different chords: C, Dm, Em, F, G, Am and Bdim. These chords correspond to the seven notes of the C major scale. In any key it is possible to build chords on each degree of the scale.
This means that for every major scale there are seven possible chords which can be used for creating keyboard parts and harmonising melodies. These seven chords are called scale tone chords. It is common practice to describe chords within a key with roman numerals as shown in the following example.
If you look at some simple progressions in the key of C major, it is easy to see how the system of roman numerals works. The following example contains the chords C, F and G. Since these chords correspond to the first, fourth and fifth degrees of the C major scale, the progression could be described as I IV V I in the key of C.
This one contains the chords C, Em, Dm and G which correspond to the first, third, second and fifth degrees of the C major scale: therefore the progression could be described as I III II V in the key of C.