A subtrope of Gotta Catch Them All referring to certain items bestowing benefits only when the player has a full or partial matching set of them. These sets are usually predetermined and have an official or unofficial collective name or a common theme to indicate that they should be used together. The resulting bonuses are often better or more difficult to obtain than those on regular items without extensive upgrades. While individual pieces of a set may be cheap (the most common ones can border on Vendor Trash at times), getting them all at once jacks up the price to a good deal more than the total cost of the pieces. Players can usually cobble together a combination of regular and/or unique items with overall stats as good as or better than a matched set of their level, but this requires more effort and resources (not to mention luck) over a longer period of time. In short, the Set Bonus mechanic offers a wholly in-game shortcut to power for people who can afford the larger down-payment. Also worth noting is that it (usually) looks better than Rainbow Pimp Gear. Most of the time there is no In-Universe explanation offered for why these items have to be together to produce the extra effects. It could be said that the enchantments need to 'overlap' or that the pieces when connected form the circuit of a system that then produces the effects. Most games with this mechanic will also have each item's description "kindly" inform the player of the potential benefits, without any proper explanation of just how the character could know about them. Maybe they were written somewhere on the items themselves. This trope is absurdly prevalent (thought by no means universal) in MMORPGs but it turns up in other genres as well. It is often used to encourage trading among players but it has the side effect of making the more obsessed sink extra time and money into a game.
Hello, Unknown Troper. You'll need to get known to lend a hand here.