To winnow is to blow something away until you are left with what you want, like grain from chaff. If you only want your favorite people at the party, you will need to winnow down the guest list from 300 to 30.
To winnow is to use air to “separate the wheat from the chafe" — literally or figuratively. If a coach winnows down the top NBA draft picks to his number one choice, then he'll focus his negotiations on that player. Chances are, if you encounter the word winnow on a given day, it is probably not in the literal context of grain.