At this stage, the U.S. House (last November) and Senate (last December) have each passed their own (different) health care reform bills. Normally, the two bodies would reach a unified compromise bill via a conference committee, and then pass the conference bill through each chamber and on to the President's desk. The win by Republican Scott Brown in last Tuesday's special U.S. Senate election in Massachusetts knocks the Democrats back down to 59 seats in the Senate, however, thus depriving them of the 60 votes needed to shut down a Republican filibuster (which a conference bill would certainly have drawn).
One possible way to salvage a bill along the lines of what's already been passed is for the House to pass the Senate-approved bill exactly as is. Because both bodies would then have passed the same exact legislation, it could go to the President without a conference process. Many House liberals apparently don't want to vote for the Senate-passed bill, though, feeling that it doesn't accomplish enough. Apparently as a result, Speaker Pelosi has said the votes aren't there in the House for the Senate bill.
Another option would be for the Senate to pass some additional provisions -- which many House liberals would like -- through budget reconciliation. Reconciliation requires only 51 votes to pass the Senate, but can only be applied to budget-relevant provisions (i.e., something that's a pure policy change with no relation to government spending cannot go through reconciliation). Nate Silver refers to potential changes through reconciliation to adjust the December-passed Senate bill as the "Senate sidecar." The House could then pass a new bill (or bills) that encompasses both the December-passed Senate bill and the Senate sidecar. Still other liberals appear to favor "blowing up" the already-passed House and Senate bills, having the Senate pass new bills through reconciliation, and having the House pass corresponding bills.
The aforementioned Nate Silver has attempted to make sense of the situation. He has a neat chart displaying a long list of specific health care provisions, how much popular support each enjoys (based on this month's Kaiser poll), whether the provision is already in the December-passed Senate bill, and Nate's guess as to whether the provision could be passed through reconciliation (due to budget relevance).