What are Downy flakes in the Stopping by Woods on a Snowy Evening?

Downy flakes in the poem just means freely downfalling snow flakes. But to fully understand the context and relevance of using this phrase in the poem, the following also are to be noted:

The sound of the horse-bells was heard distinctly against the only other background sound there, the swish-swishing sound of the easily-flowing wind sweeping against the incessantly down-falling snow. The exquisiteness of the description here reminds the readers of another master craftsman. In The Twelfth Night by William Shakespeare, there is a little song sung by the clown: 'When that I was a tiny little boy, With hay- ho, the wind and the rain.' Everyone knows the wind and the rain, but who is this Mr. Hay-Ho? Critics have long debated who this Hay Ho is. It is very simple. Every little child knows Hay Ho; it is the combined effect of sound caused by wind on the rain personified. When wind blows against a green paddy field and the long lines of grass bow their heads in row after row, Hay Ho is present there. When we walk along a tar road while the rain comes down in torrents and the wind sweeps heavily against the rain, then again we can see Hay Ho on the road, coming towards us and going away from us. Hay Ho is indeed something to a tiny little boy and also for the poets. One is always the other. An exactly similar beauty with Words is created here by Frost, in describing in vivid and suggestive Words the swish-swishing of the wind and the rain in the snow-filled forest.