In algebra, a radical typically appears in an expression like this:
In plain langauge, this means take the -th root of .
When you see the radical symbol without anything written above it, the implicit operation is to take the square root of the expression.
You can write this with a above the radical.
Another way to write a radical is as an exponent. For example, taking the -th root of an expression is the same thing as raising the expression to the power.