means Accepting something less than what you wanted, or the gradual coming-to-rest of things into a stable state.

from From the Old English 'setlan,' to seat or place, built on 'setl,' a seat or resting-place — a cousin of 'sit.' The earliest sense was literal: to put something down, to make it stay. From there it spread outward — dust settles, debts are settled, settlers settle land, nerves settle — all variations on the same idea of coming to rest. The bittersweet modern sense of 'settling for less' is a much later twist, where coming to rest quietly became giving up the search.