If you are looking for √16, you are looking for the square root of 16. The square root of a number is defined by what number multiplied by itself will get you.
x^2 (a number squared) = 16.
√16 = x.
Let's try from numbers 1-5 for a square.
1x1 = 1
2x2 = 4
3x3 = 9
4x4 = 16
√16 = 4.
I hope this helps!