squaring both sides of a trig. equation when solving - checking the roots

Hello,

So when we square both sides of a trig equation and arrive at the "solutions." some of these solutions are extraneous and arrive as a result of squaring and then taking the square root (adds the +/-sign). One straightforward way to check for extraneous solutions is to simply plug in all your answers into the original equation to see if they work.

I recently heard something about "checking the roots in the original equation" to determine which answers are valid and which are extraneous. What does "checking the roots" mean and how does one use this technique to check their answers?