Personification is a figure of speech in which inanimate objects, animals, plants, or ideas are given human characteristics. Thus, the plant (in this case) is given a kind of persona.
In "Snow," The fence posts have snow on their tops, so the speaker of the poem says they "wear" marshmallow hats. The posts didn't intend to wear the snow like a person would wear a hat. The posts just appear to be wearing hats and it is a poetic way of describing that appearance.
Likewise, the bushes aren't really wearing nightgowns and they have no knees with which to kneel and no minds with which to pray, but Aldis uses personification to make the image more poetic, alive, and more human.
The last two lines note that the trees "want" to dance away. It is a rustic, romantic notion that trees would dance away if they could but again, it is just a use of personification to give more vibrancy to the wintry scene.