Guts is an ability that increases the Attack stat of a Pokémon when they are inflicted with a status condition.
A Pokémon's Attack stat is multiplied by 1.5 when they are inflicted with a major status condition.
The status conditions that can trigger Guts are: Burn, Paralysis, Sleep, Poison, and Freeze. If a Pokémon with Guts is burned, the Attack stat is not reduced.
Pokémon with Guts