What does the albatross symbolize in the poem Rime of the Ancient Mariner?

The albatross to sailors is a sign of good luck, and
sailors everywhere for centuries are very superstitious people, so they look for signs. 
In the poem, however, the albatross is the symbol of the curse, and the origin of the
popular saying "hang an albatross around his neck".  It is a burden to be carried that
you cannot avoid or end on your own.

To kill an albatross
at sea is instant bad luck, and casts a pall over the mood of the men as they then
expect something bad will happen, and of course, the point of the story is that
something bad does happen and the crew and the boat are
cursed.