A caucus, or "partisan caucus," is basically a meeting of members of a political party (in this case, Democrats).
They do this in order to stall, and muster their last ditch efforts to gain votes. They pull all the stops here. I'm assuming there's a lot of bribery and other such under-the-table dealings at the moment.
This could last for hours and hours on end.
It could be the wee hours of the morning before we know the final outcome.
When the Assembly debated the bill the first time around a month or so ago, they were at it until around 3-4am. I wasn't too productive at work the following day.