Pre-delay is the amount of time (usually in milliseconds) for the body of a reverb to hit. Early reflections are just that - Sound from the general area. The first hits off the first walls, floors, ceilings, etc.
Big giant room, snare hit, puh-CAH from the reverb. The "puh" is the early reflections. The "CAH" is the roomy sound after the pre-delay.