A radical is a square root sign and looks like this: .
Most calculators have a on one of the keys.
If you put a number in your calculator and then hit the key, you take it's square root.
If you see the radical by itself it is assumed to be the square root that is to be taken, but this symbol can be used for other roots, for example, a cube root:∛   .
So, if you see ∛ 27, this means to take the cube root of 27, which is 3 (because 3 x 3 x 3 = 27).