Scale is a fraction of the original.

If you were building a 1/4 scale plane, 1 inch on the real plane would be 1/4" on the model So if the real plane had a 32 ft wingspan a 1/4 scale plane would have an 8 ft wingspan (4 x 8 = 32)

So if a Full scale plane had a 100 ft wingspan, and you built one with a 10 ft wingspan, yours would be 1/10 scale (10 x 10 = 100)