“Extraneous solutions are not solutions at all. They arise from outside the problem, from the method of solution. They are extraneous because they are not solutions of the original problem. ... To tell if a "solution" is extraneous you need to go back to the original problem and check to see if it is actually a solution.”
Basically, they are an answer you get when you solve a problem, but they don’t actually make the equation true so you disregard it.