Here are some examples of how it's used:
* "The lottery win was a real windfall for the family."
* "The company experienced a windfall in profits after launching their new product."
* "She received a windfall inheritance from her grandmother."
The origin of the word comes from the image of fruit falling from a tree due to the wind. It's a lucky, unexpected gain, much like stumbling upon a pile of fruit blown down from a tree.