For those not familiar we are waiting while Senators are rounded up to get to the floor to proceed with HR 3204-Missing -Senators may be in committee hearings, meeting with folks etc.

quorum - The number of senators that must be present for the Senate to do business. The Constitution requires a majority of senators (51) for a quorum. Often, fewer senators are actually present on the floor, but the Senate presumes that a quorum is present unless the contrary is shown by a roll call vote or quorum call.

from US Senate website