The la (wyl. bla) means "above" or "higher" and is sometimes translated as "spirit". The la is what remains behind when a person dies while the life-force is what ends and the consciousness is what proceeds to take rebirth.
For more informations see for example Khenpo Kartar Rinpoche, Karma Chakme's Mountain Dharma, Vol. II (Woodstock: KTD Publications, 2006), p.88.