Limits of exponential functions

I'm doing a big fat calculus review, going through Paul Garrett's [Calculus Refresher][1]. So far it's very clear and concise, but I just got stuck at one point. He lays out a bit of review on exponents (what they define, how they combine, etc.), and then says:

>  $$a^{m/n} = \left(\sqrt[n]{a}\right)^m$$