Groundout-to-flyout ratio is obtained by dividing the total number of ground balls converted into outs (not including bunts), by the total of all fly balls converted into outs.
For pitchers, GO/AO can often be an indicator of success, because a pitcher generally aims to induce more ground balls than fly balls (seeing as a fly ball can turn into the most harmful result of all, a home run). But at the same time, certain pitchers can thrive while still allowing a high percentage of fly balls, as long as they're inducing weak contact and/or striking out hitters.
For hitters, this number is used much less frequently as an evaluation tool, although speedier hitters typically strive for a higher ratio of ground balls to fly balls. That's because over the course of a season, some would-be groundouts become infield hits for batters who are fast enough to beat the throw.
Watch: Tom Koehler induces a mix of flyouts and groundouts in a quality start.