A wedding dream is one of the more universally dreamed life-event scenarios, and in most contemporary Korean interpretation it's read straightforwardly: as union, commitment, and a significant partnership forming, whether that partnership is romantic, business, or otherwise. The dragon-and-phoenix pairing covered in the phoenix article on this site is traditionally used in wedding decoration for exactly this reason — the wedding dream and that imagery share the same core meaning of two forces joining well.
Attending someone else's wedding is read as sharing in good fortune or celebration happening nearby — good news in your circle that you get to witness and enjoy, even if it isn't your own.
It's worth being honest, though, that this is one of the few dreams in this tradition with genuine historical disagreement attached to it: some older interpreters read a dream of your own wedding, much like a dream of your own funeral, with more ambivalence — treating both as symbols of a major transition or ending as much as a beginning, rather than pure celebration. Most casual, modern interpretation has moved past that reading, but it's worth knowing it exists rather than presenting the dream as unanimously simple.
A wedding that goes wrong in the dream — something disrupting the ceremony, a missing groom or bride — is read more directly as anxiety about a real commitment or partnership that doesn't feel fully settled yet, regardless of which version of the tradition you lean toward.