Meriones's bonus states:
Increases stats from a full set of Meriones's equipment by 1.5 times while in battle.
This means your full equipment set bonus parameters will increase by 1.5 when the hero is in battle.
An equipment set is considered "full" when you collect all six unique items from a given set. They may differ in quality, but they still need to be part of the same equipment set, e.g., if you combine items from the "Guardsmen's Courage" and "Specialists' Authority" sets, the bonus will not work.
Tip. Equipping Meriones with the gear you would not normally take into battle makes little sense.