Overgrow is an ability introduced in generation 3.
29 Pokémon can have this ability, such as Bulbasaur, Ivysaur, Venusaur, Chikorita and Bayleef. For 2 of them it is the hidden ability, usually obtained from events, raids or breeding.
Official effect
Strengthens Grass moves to inflict 1.5× damage at 1/3 max HP or less.