stage one: heavier pull weight and usually longer travel (take up, pre-travel, creep...whatever you care to refer to it as)than the second stage. used to 'get up on' the trigger and prepare for a calm shot release...which results in greater accuracy as a rule). a heavy first stage is also used to meet trigger weight requirements of certain classes of shooting (i.e. nra service rifle class requires a minimum of 4.5 lbs. of pul weight without breaking the trigger).
stage two. usually a short travel, crisp breaking, minimal overtravel stage that actually releases the shot.
an example of a 2-stage setup might be a service rifle with a 5 lb. first stage and a 2 llb. second stage release weight.
my rimfire match rifle is currently set up with a 2-stage trigger weighted to a 1.5 lb. first stage and a second stage that breaks at 6 ounces.
when setting up a 2-stage trigger, consistancy/repeatable feel is as critical as the travel limits and weights of pull.