The square root of 25 is 5
The square root of something is what times the same number equals something. For example the square root of 25 is 5 because 5x5 = 25
The square root of 16 would be 4 because 4x4 = 16, and so on.
Square roots that don't equal a specific number are solvable, but that's a whole different lesson.
Hoped I was able to help :)