Answer:
Absolute value.
Step-by-step explanation:
We have the expression:

The square root and the square will cancel. This yields:

We need the absolute value because any value squared is positive. The square root of a positive value will also be positive.
In other words, if we only simplified the expression down to <em>s</em> without the absolute value, if <em>s</em> was originally negative, our simplification will have also been negative.
For instance, say <em>s</em> = -7, then:

However, if we let √<em>s</em>² = <em>s</em>, then <em>s</em> = -7. By having the absolute value, we have that |<em>s</em>| = |-7| = 7, which is the correct statement.