This morning brought the biggest immigration news of Barack Obama’s presidency: Effective immediately, hundreds of thousands of undocumented immigrants brought to the U.S. as children will be granted relief from the threat of deportation and will be able to obtain work authorization. To quote Joe Biden, this is a big f-ing deal.

Immigration reform advocates, whose mounting discontent over the administration’s policy dysfunction has become a serious political dilemma for the White House, are ecstatic. Marshall Fitz, an immigration expert at the Center for American Progress, called it “huge, beyond big” and said “it’s the biggest thing that we could have possibly hoped for.” That sentiment was widely echoed: America’s Voice, a leading reform advocacy group, sent out a blast email hailing “the biggest news on immigration in 25 years,” and advocates are planning celebrations at the White House later today.

Why are advocates so excited? Because this is the most important reform the White House can make without going to a deadlocked Congress for new legislation. With Republicans blocking even the most trivial legislation, no one has any realistic hope for a comprehensive immigration reform law in the near future. Even the DREAM Act, which passed the Democratic-controlled House in late 2010, was blocked by a minority in the Senate (55 votes in its favor were not enough).

But this policy will provide relief to so-called DREAMers: people who were brought to the United States at a young age; who count America as their home country; who, though here illegally, are not here as a result of intentionally breaking the law. They will now be able to come forward and apply for deferred action, which will grant them relief from deportation for two years. They’ll also be able to apply for work authorization, so they can support themselves as well. The policy may affect as many as one million undocumented immigrants who would have been eligible for the DREAM Act. After the two years are up, they can reapply for an extension of those benefits.

An announcement from the Department of Homeland Security outlines just who is eligible for this relief. Applicants must have come to the U.S. before turning 16; they must not be older than 30; they must have continuously resided here for five years; they cannot have committed a felony, serious misdemeanor, or multiple misdemeanors; and they must be in school, have graduated high school, obtained a GED, or be an honorably discharged veteran. In other words: This is not a blanket policy for anyone who arrived here as a child—applicants must demonstrate some merit. Nor, as Secretary of Homeland Security Janet Napolitano emphasized this morning on a press call, is this “amnesty.” The policy does not provide a path to citizenship or legal permanent resident status. “It is not immunity; it is not amnesty,” she said. “It is an exercise of discretion.” Immigration policy, she argued, is “not designed to remove productive young people” to countries they don’t know.

And while earlier moves by the administration on immigration have brought disappointment, this policy could actually mean relief for hundreds of thousands who deserve it. First, the directive is written in clear, strong language—and it appears to actually be more of a directive than a “recommendation.” In the past, discretion has been, well, discretionary, and non-compliance among field-level immigration officers has hampered attempts by headquarters to change enforcement. This time, there seems to be a firmer, more mandate-like approach from the leadership. (Though, to be sure, people will be watching closely to make sure the policy is implemented.)

Second, this is an affirmative process: Anyone, including those who have never been subject to enforcement action, can come forward and apply for these benefits. Prosecutorial discretion, as I have documented, is something that people who are already in the midst of enforcement actions request of Immigration and Customs Enforcement (ICE)—the agency responsible for enforcing the law. But this process will be handled by U.S. Citizenship and Immigration Services, which is the agency that handles benefits, not enforcement. The culture and mentality are different, and the affirmative nature of the process means that the emphasis will be on granting these benefits whenever possible, to whoever comes forward.

There is much more to say about this, and I’ll have more commentary in the days to come—and perhaps later today, if the President makes more news in his remarks at 1:15. I haven’t even addressed the possible electoral implications of the move (a question I’ll leave to TNR’s in-house campaign whizzes). But from a policy angle, and from a humanitarian angle, today’s news is truly monumental. As David Leopold, former president of the American Immigration Lawyers Association, told me this morning, “The president has used his authority to stop the deportation of promising young DREAMers while giving Congress space to fix the system.” It’s a bold move—one that has already drawn a sharp response from many Republicans, and which will doubtlessly be controversial. But for nearly a million young people who call this country home, today’s news means something much more fundamental: relief from the nightmare of deportation and the chance to earn a living without hiding in the shadows.