Anchors Aweigh

The translator from Poland is back with a long series of questions concerning another novel.
To start with, here is a song:

http://www.geocities.com/bascott99/anchorsaweigh.html

What does "anchors aweigh" mean (in this piece)? The anchors are close to the bottom, from what I understand. Are they leaving and raising the anchors totally or is it - on the contrary - just a preliminary maneuver, maybe done the evening before they leave?