Partner is a Commander deck-building ability. It lets your deck have two commanders instead of one, as long as both commanders have partner. The basic reminder text says you can have two commanders if both have partner. Some cards use a labeled version, such as Partner—Survivors or Partner—Father & son; those say you can have two commanders if both have this ability, so the two commanders must share that same labeled partner ability.
In play, Partner widens your options. You can combine two partner cards to gain two color identities, two sets of abilities, and more consistent access to the effects your deck wants. Your command zone starts with both cards, so you build around them together and can cast whichever one suits the game.
