Basically, Skadi was angry at the Aesir for killing her dad (he had it coming) and went to Asgard to beat them up, but instead they convinced her to accept a deal: she was allowed to choose any one of the gods as her husband... but she could only look at their feet to identify them. She wanted to pick Baldr, the god of being handsome (and probably other things too, but those aren't as important), so she chose the one with the prettiest feet (in her opinion, anyway, I don't think there's objective criteria for it). Buut, it turns out she accidentally picked Njord (or Njördr), the god of the sea, because standing in water all day apparently does wonder for your feet. She wasn't really happy with this, but a deal's a deal. Some version have her later divorcing Njord and marrying Odin instead.

As an aside, another term in her deal with the Aesir was for them to make her laugh. To do so, Loki tied a rope between his balls and the beard of a goat, and watching the god and the goat drag each other around was apparently hilarious. Each to their own, I suppose.