What it does
Partner lets you choose two legendary creatures as your commanders instead of one, as long as both have the Partner ability. Both go in your command zone at the start of the game, and both contribute to your color identity—which is the main appeal, since you can build, say, a four- or five-color deck from two two-color partners.
The biggest misconception: Partner doesn't combine the two creatures or let you cast both for cheaper—each is cast separately and carries its own commander tax independently. Note that "Partner with [name]" is a different, more restrictive ability that only pairs with one specific creature. Generic "Partner" pairs with any other generic Partner commander.
In Commander, Partner is powerful for color flexibility and redundancy; if one partner is removed repeatedly, you still have the other to apply pressure or enable your strategy.
