Pre-delay is the amount of time before you start hearing the reverb signal. If you were standing in an empty room and shouted, the pre-delay would essentially be determined by the size of the room. A smaller room would have a short pre-delay while a large one would be longer.

Decay is essentially the reverb time. It determines how long the reverb will linger in the sonic field.

Tone determines, well, the tone of the reverb. Generally, a higher tone setting will give you a brighter reverb sound.

Mix determines how much reverb is mixed in with the dry signal. The dry signal remains at 100% throughout the travel of the parameter except at the very end. At 100% mix, there will be no dry signal just reverb.

This page explains everything in more detail: http://www.retrosynth.com/~analoguediehard/studio/effects/digitalreverb.html