Leis are super-pretty tropical flower necklaces—in our opinion, they absolutely trump flower crowns. They're iconic of Hawaii, luaus, and (thanks to this movie), uncertain war-torn romance.
At the end of the movie, Alma and Karen Holmes are on a boat leaving Hawaii. Prewitt has been accidentally shot by his own men, and Sgt. Warden didn't want to become an officer and try to leave Hawaii with Karen Holmes. So they're both fairly disappointed.
As they drop leis into the ocean, Karen Holmes says,
"There's a legend: if they float in toward shore, you'll come back some day. If they float out to sea, you won't."
Now, according to the screenplay, the garlands float out to sea—so, technically, the women never come back (if this legend is even true). But lots of things from the screenplay were changed before they made it to the screen, and we can't really tell where the garlands are floating. It's left up in the air. Who knows what will happen to these characters after the war ends… or even while it's happening?