President Trump is expected to sign the measure by the end of the week.

The law is controversial because the surveillance program, although aimed at foreigners, has resulted in the U.S. government monitoring the communications of Americans—including email and chat histories—without the sign-off of a judge. Proponents say privacy concerns are overblown and the program is an essential anti-terrorist tool.

But what exactly does the law, known as Section 702 of the FISA Amendments Act, allow the government to do? Here’s a plain English explanation.

What does the law allow the government to do?

It forces Internet companies like Google and Facebook to share data with the agencies and let them carry out searches of their users’ communications. The data includes everything from texts to email to browsing history. In practice this means the NSA and FBI collect and search a massive database containing hundreds of millions or billions of records.

A second part of the program authorizes the spy agencies to tap into overseas Internet cables run by companies like AT&T, and extract phone and data communications.

How can they do all this without a warrant?

Normally, the government would have to get a judge’s permission by showing probable cause to carry out such a search. But the ostensible targets under Section 702 are not Americans—they’re foreigners who don’t enjoy the same Constitutional protections as U.S. citizens.

Why is this is a big deal if it’s aimed at foreigners?

The law is controversial because it’s easy for the contents of Americans’ communications to get swept into the NSA database. For instance, a target of an overseas investigation may email a U.S. citizen. Critics also claim that the process is prone to “reverse targeting” in which a government agency picks an overseas person as target even though their real purpose is to eavesdrop on an American who that person might be in contact with.

Privacy hawks also fear the current process lets the government, when it taps into the Internet cables, specify an American’s email address or phone number for searching simply because it has come up in conversation between foreign targets. They also warn that the criteria for using the surveillance—national security—is broad that it can sweep up all sorts of activity.

How long has all this been going on?

This has been happening since the Bush Administration, though the extent of the surveillance only really came to light following the dramatic 2013 leaks by former NSA contractor Edward Snowden.

So why is the latest Senate vote significant?

The Senate’s decision to approve the bill, which passed the House last week, means the existing surveillance procedures are set to continue basically unchanged for another six years. While the law contains a new provision that requires the FBI to get a warrant if it wants to use the Section 702 database in an ongoing criminal investigation, most observers say the new rule contains too many exceptions to make a practical difference.

Who supports the law and who opposes it?

Unlike most bills in Washington, the 65-34 vote in the Senate did not fall on strict party lines. Opposition to the law came from a coalition of Democrats, led by Sen. Ron Wyden (D-Ore.), and from libertarian-leaning Republicans including Rand Paul (R-KY).

Meanwhile, security minded Democrats, including minority leader Nancy Pelosi (D-Ca), joined the majority of Republicans in supporting the law. Many in law enforcement, meanwhile, claim the law is an essential means of preventing terrorists attacks, and claim the privacy concerns are hypothetical or overblown.

Is there any oversight to prevent spy agencies from abusing the law?

There is a check on the NSA and FBI in the form of the secretive spy court known as the Foreign Intelligence Surveillance Court, which periodically approves rules about how the spy agencies collect Section 702 intelligence.

Those rules concern so-called “minimization” procedures to ensure Americans do not become the target of unconstitutional surveillance, as well as requirements for the spy agencies to destroy data after a period of time.

The court’s most recent ruling (much of it redacted) came last May when it approved new NSA safeguards to limit the incidental collection of American citizens’ data. While the court does provide a form of legal oversight, critics of the law say it is not enough, and had proposed a series of amendments that would require the government to seek a warrant whenever a Section 702 investigation affected an American.

Where can I read more about Section 702 and privacy?

The non-profit group, Center for Democracy & Technology, has a very readable overview of the surveillance practice and the legal rules that surround it.

To get a flavor of why critics consider the law as unconstitutional, see these posts from the Electronic Frontier Foundation. For the other side’s view, a federal prosecutor explains here why he views Section 702 as both necessary and constitutional.