A loser is the team or person that did not win or succeed. In pond hockey, the loser has to hose down the ice to make it smooth again.
You may have heard loser used to insult someone who has not had a lot of success in life, someone who might not have many friends. This mean slang took root in the 1950s but it wasn't until the 1990s that kids who loved indie rock reclaimed it as an anti-hero badge of honor, wearing t-shirts with "Loser" written in huge letters across the front. This ironic gesture was meant to show the jocks who the real cool kids were.