A persistent world (PW) is a multiplayer module that runs continuously with plots and events evolving as time passes. They often fit into game type categories like "story", "roleplay", "action", etc.
All persistent worlds are run by players of Neverwinter Nights. BioWare was interested in the potential of persistent worlds, but they chose to take Neverwinter Nights in a different direction and never had official plans to develop a persistent world. Instead they provided tools (primarily the Toolset) and saw what their fans would come up with.
What I'd like to see in the distant future is eventually persistent world support, similar to what Bioware did with NWN, catering to 20-40 players at a time. I've no idea if one would be able to do it with the toolset, but I think it has a lot of potential. There's some issues like how to resolve turnbased combat with a large group of players, but it could be done if you implemented things like pre-issuing orders for your PC when it's not your turn, and a set timer on decision-making. It may add some further longevity to the game beyond what the toolset already provides.