Andrast was a long cape in the south-west of Gondor.
The cape was nominally part of Gondor, but it was not populated by the Númenóreans. At the end of the Third Age, it was believed the area was still populated by tribes of the Drúedain and other hostile men. The Drúedain were notorious for using poisoned arrows.
The White Mountains ended their southward bend at the Andrast, and as such it formed an effective barrier to invaders from the north for Gondor. There were no passes over it, so all had to venture around the Ras Morthil.