If you have a basic knowledge of Sub-chapter S corporations and Limited Liability Corporations (LLCs) and how taxes and deductions on those organizations are 'passed thru' to the actual people (often small business people) it will all make sense.  

The Democrats thought that they would get more money out of a tax break for  some of these organizations; that didn't get them the money they wanted, so they want to renege on the deal they made to give these breaks if the Republicans would back their (ultimately illegal) scheme to cut pension benefits for already retired people.