Numbers indicate notes in the sequential order, starting with the root note. E.g., CDEFGAB == 1(C) 2(D) 3(E) 4(F) 5(G) 6(A) 7(B). The idea is that the intervals between notes in any key is the same and numbers give you a way to think in intervals and patterns no matter what key you're playing. E.g., if your minor triad is C Eb G, then you "phrase" it as 1 3b 5 and it means that in any key root, 3rd note flat and 5th note will be the minor triad

Hope it makes sense

Dima