Thanks in large part to Sen. Rand Paul, at midnight on Sunday, the U.S. Senate let the Patriot Act expire, removing key controversial surveillance authorities from the National Security Agency. So what does that mean, and what's next?

The inner workings of Washington, D.C. are incomprehensible enough for most. Add to that esoteric subjects like court-approved surveillance authority, digital technology, and constitutional law -- essential to the sensational story of the Patriot Act's non-renewal -- and you'd be forgiven for any subsequent eye glazing. Here are the basics on what's happened, why it matters, and what will likely happen next.

What Happened

The USA PATRIOT Act was a giant anti-terrorism bill the Bush Administration rushed through Congress just weeks after September 11. Over the years, it's been renewed by Congress several times, and under President Obama in 2011, a four-year extension was passed that extended key portions used by the administration to authorize National Security Agency activities. That had an effective expiration date of June 1, 2015.

Some of those key sections of the 2011 Patriot Act extension were used to authorize NSA surveillance, including the very controversial domestic, warrantless, meta-data (or phone records) collection program exposed by NSA whistleblower Edward Snowden almost exactly two years ago.

Flash-forward to about a month ago, and after an avalanche of revelations about the NSA's power and intrusiveness, the political willpower to renew the USA Patriot was clearly waning.

Then came a big court win for surveillance opponents in the first week of May. A three-person panel of federal appeals court judges ruled that the NSA's domestic activities exceeded "the scope of what Congress has authorized" through the Patriot Act, specifically ruling that the legislation "does not authorize the telephone metadata program."

Finally, late last week, libertarian Republican (and candidate for the GOP presidential ticket in 2016) Sen. Rand Paul threatened to stop the already behind-schedule vote to renew the Patriot Act, saying, "I will force the expiration of the NSA illegal spy program."

Against some Democrats and his own party's Senate Majority Leader Sen. Mitch McConnell, Paul flexed his 'Mr. Smith Goes to Washington' muscles late Sunday on the Senate floor for 10 hours and forced the Patriot Act to expire.

Why It Matters 

The expiration of the USA Patriot Act strips the NSA of its power to collect bulk phone records on all Americans, easily the most controversial element of NSA programs exposed by Snowden.

That program is likely never to bounce back, since the USA Freedom Act, which will likely be the Patriot Act's replacement, also forbids it.

Along with the NSA's metadata program, CIA and FBI authority to monitor "lone wolf" terrorist suspects -- that is, people who have no provable ties to a terror network or terrorists, but are monitored anyway -- and the authority to conduct "roving wiretaps," which target individuals rather than specific phones, are all off the table since 12:01 Monday.

The big takeaway from this is that pressure from politicians, organizations, and citizens has managed to take down a more-than-a-decade secret spying program, likely for good.

It's a big win for privacy activists, limited-government advocates, and Edward Snowden, without whom, as The New Yorker recently noted, none of this would have likely happened. He remains in exile from U.S. authorities in Russia.

It's also a big win for GOP presidential hopeful Rand Paul, whose central role in the Senate's failed renewal bid will almost certainly boost his appeal across party lines with those who value privacy and/or understand how much digital metadata can tell about a person in the modern world -- namely, millennials.

No wonder he tweeted before Sunday's big showdown, "I will take down illegal NSA spying. Contribute $5 now to stand with me."

What's Next?

Sen. Rand Paul won't be helping, but it's very likely that the USA Freedom Act will pass before the week's end -- depending on amendments to the bill. Already late Sunday, the Senate overwhelmingly moved to vote up or down on the Freedom Act, 77-17, according to USA Today.

That's a supermajority that overrides any specific, outspoken Senator's filibuster attempts. The New York Times has predicted the bill will pass by early afternoon, Tuesday.

The upside for NSA critics is that the USA Freedom Act was created to counter the NSA's most intrusive policies, including the metadata program, and to reform the NSA's use of the FISA court as a completely non-transparent rubber-stamp warrant factory.

The downside for advocates is that many have criticized the bill for various loopholes and doing too little in its reform mission.

One of the most prominent privacy advocates and critic of the USA Freedom Act, the Electronic Frontier Foundation, for what it's worth, summed up its position on the next step for Congress and the NSA in this way: "A step forward is better than a step backward."