What you see are the catkins, or the male flowers, of an oak. Oaks produce both female and male parts and are wind-pollinated, which means they don’t need insects to reproduce. The catkins are full of pollen when they are still on the tree. Once their pollen has been released into the wind, seeking the female flower (small and hard hard to find),  they fall from the tree.  What you see are the spent flowers. If you don’t see them every year it’s because that particular species of oak produces acorns every two years. Other oak species reproduce annually. These catkins do no harm but make for an unsightly landscape for awhile.