## need help figuring out the meaning of this congruence exercise

From Abstract Algebra, 3rd ed. (Beachy, Blair), p32 (section 1.3):

2. Write $n$ as a sum of four squares for $1\leq n\leq 20$.

What exactly is this asking? The section has to do with congruences and modular arithmetic, but I don't see anything in the chapter that clues me in on this particular exercise.

Any help would be much appreciated.