A reserve feat is a feat tied to a type of spell, and it allows you to do magic at no real cost to you indefinitely. For instance, there is a reserve feat that allows you to cast a fireburst that does 1d6 per the level of your highest memorized fire spell. So if you were a 5th level wizard with fireball, as long as you didn't cast your fireball (and get 5d6 fire damage in a huge radius) you could every round do a mini 3d6 fireball in a small radius. There were feats for lots of types of spells, healing, summoning and teleportation being the most memorable to me. My DM banned them for being to powerful, so I don't know how they play.
Jay