"Like" radicals can be added and subtracted in the same way any like terms can be combined. It can be helpful to simplify the radical as much as possible so that it can be seen whether the radicals are "like" or not.
Examples:
√2 +√3 . . . . cannot be combined
√2 +√8 = √2 +2√2 = 3√2 . . . . the simplified radicals can be combined
