Other forms: winnowed out; winnowing out; winnows out
To winnow out is to eliminate undesirable things, leaving only what you want, like when you winnow out old or stained t-shirts from your wardrobe, so that you're left with a drawer full of stylish and up-to-date tops.
The early stages of a spelling bee serve to winnow out the struggling spellers, leaving only the serious competitors behind. And when a gardener thins her crop of tiny carrot seedlings, she winnows out the garden bed, disposing of the tiniest plants so the bigger, stronger ones have more room to grow. This phrase comes from winnow, "remove chaff from grain."