A "soft opening" is when the WWoHP will open it's doors without necessarily advertising the fact that it is ready/open. Not all rides, shops might be open. The time allows them to test the operation of things, train up staff, monitor queues etc etc. If you are in the park during this time you might get to experience the new area (almost a guinea pig!) as a bonus. If things are not working or have to close again for short periods the park in a way avoids any potenital customer disappointment as it has not advertised the WWoHP as officially open, just if you are there and it is operating it's a bonus for you.
Hotels sometimes have "soft openings" when they have just opened their doors and want to test things out. Staff might be new and being trained (or even finding their way around the building literally), not all facilities are open, some finishing work like painting might still be going on and rooms will be tested for the first time for guest satisfaction (if the shaving mirror in the right place, does the bedlinen wash well, how do the restaurant staff and servers cope feeding xxx people at the same time etc). Often hotels will offer very special deals to those in the travel industry to stay and try things out - or open up to the public with special introductory rates.