what is a widow's walk?

I am trying to explain this metaphor's meaning in the book, but I don't understand this term. Can any one help please????
THX A LOT!!!

From www.answers.com:

"A railed, rooftop platform typically on a coastal house, originally designed to observe vessels at sea."

The reference is often to women who kept watching for their husbands to return from a sea voyage ... but never did. In olden days (no radios or satellites), if a ship was lost, it was simply lost. No news.