Soft opening is usually when the ride opens unannounced for visitors to discover it and ride it so that if there are any kinks inthe system, they can be worked out before the official opening. Supposedly, during soft openings, the ride can close at anytime if need be.
It'll be interesting to see how this works with M:S since soft openings are not usually announced. This one was, which makes one wonder if a lot of testing has already been done and they are anticipating little if any problems.