Basically, if the intrinsic value of a stock is $20, but the stock currently trades at $5, you would be looking at a $15 gain or 300%. This means the stock was undervalued.
And the opposite would be true, if the stock was trading at $20, but it's intrinsic value is $5, the stock is overvalued and not a bargain.
There are a few stock intrinsic calculators out there on the web. Some are more complicated, but there are easy ones where you can just enter the stock ticker and hit calculate.
There's also websites like gurufocus (free to use, although there's a pestering pop-up if you don't sign up for a free account) that will do the work automatically, with their "Fair Value Calculator."