Come here hoser, I was just kidding.
Used as a negative, it basically means "idiot." When used positively, it's a phrase of light endearment.
In both cases, it is most effective when the subject(s) displays stereotypically Canadian attributes.
POS - "They drank all our beers and smoked all our smokes, but took the ski-doos to get more before the hockey game started. What a bunch of Hosers."