Like Blaze, Torrent, Swarm etc.
If your Pokémon has less than 1/3 HP, it's STAB on basic type will become +125% instead of +50%.
Blaze - Fire STAB
Overgrow - Grass STAB
Torrent - Water STAB
Swarm - Bug STAB
Those type of abilities don't activate until your pokemon's HP falls below 1/3 its maximum. If my Typhlosion has 300 HP, and he falls below 200, then blaze will activate. It powers up your pokemon's move of that type, so it's like having a double STAB.