Well it means what key the piece is in.
1 sharp- G major
2 sharps- D major
3 sharps- A major
4 sharps- E major
5 sharps- B major
6 Sharps- F major
Now while this applies to sharps, it is reverse for flats
1 flat- F major
2 flats- B major
3 flats- E major
4 flats- A major
5 flats- D major
6 flats- G major
I remember it as GoD AtE BeeF.
Or if you want to know why it works, it's because it goes up in the circle of fifths, starting from C major. So C is the 1st, E is the 3rd of C and G is the 5th. Once you have G major to find the next key you will again have to find the fifth, so G is the first, B is the 3d and D is the fifth
Please note this doesn't take into account of accidentals, A tonal music or work for minor keys but it applies to the majority of conventional music
Hope this post helped
If you need any more help PM me, I will happily respond