It's just a shorthand way for the studio players to remember what to play when, instead of writing down chord names, Frinstance, if you're going to be playing something in the Key of C, you'd begin by knowing what chords are part of the C scale, namely C - Dm - Em - F - G - Am - Bdim7 - C. You begin counting at 1, beginning with C, so
C= I
Dm = ii
Em = iii
F = IV
G= V
Am = vi
Bdim7 = vii
They're always written that way, in roman numerals, with the I, IV and V written in caps, and the ii, iii, vi, and vii written in lower case. I dunno why...
It also helps if you're transposing, or want to use a capo and don't want to write out another whole chart.
Say frinstance your bud is playing a song in G with the chord progression
G - Am - C - D, you'd write the chart as I - ii - IV - V .
Now, you want to capo up three, so you can use that fancy lick you learned to do out of an E fingering. Remember, he's playing with his fingers in the G position and your capoed up three frets playing in an E position... all you have to do is count which are the chords equivalent to I - ii - IV and V in E, so you start counting with E as I, ii is F#min, IV is A and V is B
If you're playing your IV and he's playing his IV, your fingers are in different positions but you're playing the same thing tonally. Is that clear at all? Probably not, but that's how it works.
It's fun to look at a chart like this after a studio session, there are all sorts of other little squiggly things that mean 'come in early here' or 'retard this last part'.
But basically it's just a musucian's shorthand.
Yoda
