Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build focuses on maximizing Garen's killing ability through increasing the three main attributes that I feel help him stand out.
First, attack damage is vital - this really helps Judgment shine as your most powerful ability, and it can be DEVASTATING early game if you're running with a stun-capable player. Second, critical chance also helps maximize the damage that your Judgment causes as well as helping keep your normal melee attacks hurting the opposition. Third (and some would argue, not finally) is attack speed. This last stat is really up to interpretation, and you could probably replace it with health for survivability or magic resistance/armor to counter-act other heavy DPS.
Focus on attack damage first, then critical chance. After that, gauge your game and see what stats would help you best.