If you’re unfamiliar with the concept of Three True Outcomes, it started as a joke by Christina Kahrl and makes reference to the idea that hitters can only control three outcomes in any plate appearance: they can strike out, work a walk, or hit a home run. Any other hit is determined by defense, and who cares about that?
It’s mainly just a fun idea, as it tends to highlight absurdist player careers like Russell Branyan and Adam Dunn (and, now, I suppose Joey Gallo), who are basically no average, all power, all strikeout, all walk-style hitters.