The Pokémon can use "not very effective" moves to deal regular damage.
Tinted Lens (Japanese: いろめがねTinted Glasses) is an Ability introduced in Generation IV.
Effect
In battle
Tinted Lens doubles the damage done by any moves that the Pokémon uses which are "not very effective". This effect is independent of whether the type effectiveness is ½× or less.
If the Pokémon uses a move that is not very effective, the move's power goes up.
Trivia
This Ability is probably a reference to the idiom "viewing the world through rose-tinted glasses", an expression which means to see good where there is none.