An extraneous solution is a solution that's not really a solution. In other words, you work out the problem and you obtain a solution. Then you test your solution by plugging in your solution to the formula given, if the result is false then it's considered an extraneous solution.
For example, if you're solving for time it took for a football to reach the ground after somebody threw it, you would obtain two answers for time. These time answers are negative time and positive time, well the negative time makes no sense since it's before you threw the ball so we would consider that negative solution to be extraneous.
