It shows the relationship to tonic notes.
Place the pointer Tonic major next to C and read off:
eg: minor tonic = A
fourth = F
fifth = G
and so on.
If you wanted to play the relative minor chord, you're in F; the chart will show that Dminor is what you're after.
If you don't know what to add to get a Cmaj7 chord, the chart shows you the major 7th in C is B.