Political class is in session again and here's this week's topic: just what is a "rider?"

The so-called "riders" are key the heated battle that is holding up funding of the government right now.

No, it's not lawmakers riding on motorcycles or horses, but rather legislative speak for an amendment added to a bill that's different than the bill's original intent.

Um, but what does that mean in plain English though?

Well, actually -- let's get back to horses -- which provide a pretty good way of understanding it.

Fox News' Senior House Producer Chad Pergram puts it like this: If you take a horse and put something on top of it, the horse is riding -- just with something additional stuff on top of it -- but it's still riding. Of course, the horse can still run without the attachment, but the add-on can have an impact.

Just like a jockey can make a horse ride better with different techniques and sizes, "riders" in bill can have the similar effects. In a way, the add-ons can sweeten the deal, but also alter the bill from original version. Those initially supporting it may not like the altered one, and it's about getting the mix just right.

So how are the riders holding up the government being able to run?

Fox News' House Producer John Brandt explains: The most contentious ones so far have been a provision that would prohibit any funds in the spending bill from going to implement President Obama's health care legislation, and a provision to prohibit the EPA from regulating greenhouse gasses.

Senate Majority Leader Harry Reid, D-Nev., calls the final sticking point a provision that eliminates federal funds from going to Planned Parenthood. Republicans say it's because the group performs abortions. Democrats say that money goes to Planned Parenthood for Title X funding, which is for women's health and that federal funds in appropriations bills can't go to fund abortions due to the Hyde Amendment.

Despite Reid's claim that the only sticking point is "women's health, Boehner's office responded, "While nothing will be decided until everything is decided, the largest issue is still spending cuts. The American people want to cut spending to help the private sector create jobs - and the Democrats that run Washington don't."