What does "empty nest" mean?

The home of parents whose children have grown up and moved out. For example, Now that they had an empty nest, Jim and Jane opened a bed-and-breakfast. This expression, alluding to a nest from which baby birds have flown, gave rise to such related ones as empty-nester, for a parent whose children had moved out, and empty-nest syndrome, for the state of mind of parents whose children had left. [c. 1970]