You should understand what it means to square a number, and understand that square rooting is the inverse (opposite) operation. You should recognise square numbers up to 15 x 15.
When you square a number, you multiply that number by itself. You will often see a little "2" written at the top-right-hand corner of the number to show you are squaring it. It is very important to remember that this little 2 means "square" and not "multiply by 2"!
When a number is written like this, it shows that you are raising the big number to the power of the little number. So we could say "to the power of 2" rather than "squared". These little numbers are also known as indices or powers.