Rime is white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, and any other solid objects.
It looks quite similar to hoar frost, but rime ice is formed by vapour first condensing to liquid droplets (of fog, mist or cloud) and then attaching to the surface, while hoar frost is formed by direct sublimation from water vapour to solid ice. Sometimes the rime ice takes on a feathery look and looks very much like "snow feathers".
Here is an article that explains the value of our extreme weather warnings.