Story highlights The National Defense Authorization Act authorizes levels of defense spending

Sen. John McCain is piloting the must pass bill and vote is slated for Monday

Washington (CNN) The Senate is set to move forward with its version of the annual defense policy bill known as the National Defense Authorization Act -- marking the latest step for a key piece of legislation that has been passed by Congress for 55 straight years.

At a time where passing bills is often challenged by the deep partisan divide and competing political factions on Capitol Hill, it can be difficult to understand why the NDAA would be any different.

But as the measure that sets military policy on issues such as Guantanamo Bay, buying weapons, pay raises for service members and even the endangered status of the Sage Grouse, the NDAA is considered of one the few must-pass bills left in Congress.

So what makes it so important?

One of the most obvious reasons is the money.

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