1 sf is the area represented by a square that has a length of 12 inches (1ft) and a width 12 inches (1ft) This area does NOT have to be a square, it can be any 2 dimensional shape.
1 sf could also mean 1 significant figure. That is a value rounded in such a way that it contains only one non-zero digit. For example:
4321 = 4000
21.34 = 20
0.0063 = 0.006