humming.(hum)v.t.to sing with the lips closed.

Humming is a vocalized sound made with the lips closed.
Humming is used for relaxing and utilizing some of the muscles involved with singing, although humming is rarely an actual part of a song. To hum as part of a warm up exercise helps keep the emphasis off the throat area and in the mask of the face opening the nasal passage and lifting the soft palate.
When we practice our humming it helps if we smile as we go higher in our range, we lift the muscles in the face. It is far more likely that our lips are relaxed if we are smiling. It also helps keep our focus in the mask of the face and opens our resonators (the cavities in the nose mouth and throat and chest which amplify the voice).