Your Avatar rating is the amount of Quintessence you can hold in your Avatar without knowledge of the Prime Sphere. All you have to do is to go to a node and meditate there for at least an hour. Each hour, you roll Perception + Meditation (Diff. 7) to refresh your avatar, refilling one point of Quintessence per success, up to your Avatar rating. Your Avatar is also the maximum amount of Quintessence you can spend in a turn.
Nodes are where Quintessence gathers naturally (or sometimes is made to gather). You can collect the Node's rating in Tass per week, in some form appropriate to the Node. You can also meditate there to refill your Avatar with Quintessence. Nodes can be used by Prime weilders to absorb Quintessence beyond their Avatar rating, and are also used to help power Horizon Realms.
Periapts are special containers which have been prepared with Prime 4 to store Quintessence. A periapt holds five points of Quintessence for every success obtained when creating it. (If you have the Quint to put in it, anyway). Prime 3 is used to actually transfer the Quint. in and out.
Quintessence is the fundamental building block of reality. In game terms, you can spend up to your Avatar Rating (or your ranking in Prime, whichever is higher) in Quintessence on an effect to reduce the difficulty. The use of Quintessence has a few side-effects:
Tass is Quintessence stored in some sort of object; it usually has strong resonance, and is produced by Nodes. Tass can only be detected and used properly if you have at least one pip of Prime.
What can you do with the Prime Sphere? (I'm only listing up to
the limits of what anyone in the game actually HAS)