Side quests are those that you get from someone other than a person in the chain of Main Quests. For example, in Grizzleheim, Snorri Oysterbloom gives a side quest -- absolutely nothing to do with anything else going on there.
After you beat Skellek, you need to level up some more for the next area to open. You'll get a message at level 25 to go see Merle Ambrose and he'll tell you to go talk to Bjorn Ironclaws again. Go back to your questing in another world and get more experience.