Collier is a term usually used for the owls who know how to capture or find coals.
Colliers are said to be descended from the first colliers, Fengo and Grank. Grank would fly into the Sacred Volcanoes and get coals that he would use to heat metals. Fengo helped him find which ones were good coals and told him of the Ember of Hoole, a powerful ember which was used to traveled around the five volcanoes.
As said above, the word "collier" is mostly used for owls. However, some of the first colliers were wolves. Colliering owls usually find their coals in/around the Sacred Volcanoes, or they fly into forest fires for them.