Boosts move power when used consecutively. (Hold)
Metronome increases the power of a move each turn it is used consecutively, going up to a maximum of 100%. This will reset if you use a different move or switch out, though.
This item is interesting for Pokémon with one dominant STAB move they spam repeatedly. After a few turns of using the same move, the damage boost becomes significant. However, being locked into one move is risky and good opponents will exploit the predictability.