Origin refers to the beginning point or location. In any coordinate system, the origin is expressed as 0,0,0. Origins are important. If you layout your scene or rig your character properly, you can animate with numbers that make sense (such as returning a rotation to 0 to get back to your starting position). One of the benefits of using nulls in hierarchies is that by parenting your object to a null, you can keep the global animation moves on the null, but all other moves locally to the object and its sub-hierarchy. This will allow the sub-hierarchy to be easily reset, and yet still allow for precise locating in world space with the null.