Hi bioschip,
It is easiest to explain with examples.
square root of 9 is written $$\sqrt9$$ and it equal $$3$$ because $$3\times 3=9$$
The opposite of squaring a number is taking the square root of the number.
For example $$5^2=25$$ and $$\sqrt{25}=5$$
$$\sqrt{64}=8$$ because $$8\times 8=64$$
Do you get it now?