You may want to review powers first.
The nth root of a number x is the number whose nth power is x. For example, the fourth root of 16 is 2, since 24 = 16. The second root is also called the square root, and the third root is also called the cube root. We write the nth root of x like so: n√x. If we want a square root, then we don't have to write the 2; we just write √x.
Note that a number might have more than one root, or none at all. For example, −4 has no square roots, while 4 has two of them: 2 and −2. When we write √4, though, we will always mean the positive root, 2.