Here's that square, drawn exactly to scale. Each of the
little square units is 1 square centimeter.  Count them 
and you will see that there are 49 of them: 



Now if you look at how many centimeters there are in
each side, you will see that there are 7 centimeters
in each side.  That's because the square root of
49cm² is 7cm.  And that's true because 7² = 7×7 = 49.

Edwin