The White House decided this doesn't count as being in session and went forward with its recess appointments. This was challenged, which is what today's ruling is about. As usual, SCOTUSblog's Tom Goldstein has the clearest, concise rundown:

The President can make a recess appointment without Senate confirmation when the Senate says it is in recess. But either the House or the Senate can take the Senate out of recess and force it to hold a "pro forma session" that will block any recess appointment. So while the President's recess appointment power is broad in theory, if either house of Congress is in the hands of the other party, it can be blocked.

That means a sharp limit on this, or any, president's ability to appoint federal officials going forward. As Epps reported in January, Miguel Estrada, arguing for Senate Republicans, suggested that the Court let the Senate decide when the Senate is in recess, and that's what the justices did. The recess-appointment power stands, but it ain't what it used to be.

The decision is bad news for Obama. His administration had argued for an expansive view of recess appointments, and it got slapped down by all nine justices. But some conservatives had hoped for a more sweeping restriction on the executive branch's recess powers.

2. What does this mean for the NLRB?

First, the decision means Noel Canning is off the hook for the NLRB's decision, which found that the company had failed to write and execute a collective-bargaining agreement. It also may mean that any decision the board made in other cases while the recess-appointed members were on the board is now invalid.

3. What does this mean for politics right now?

In the immediate term, it's not clear that this will make a big difference. What precipitated Obama's NLRB appointments was a standoff in the Senate between Obama and Senate Democrats, who wanted to confirm nominees, and Senate Republicans, who vowed to block them. That crisis came to a head when Majority Leader Harry Reid invoked the "nuclear option," lowering the threshold on confirmation votes to a simple majority and eliminating the threat of a filibuster. So even though Republicans still control the House, Obama hasn't had to rely on recess appointments to fill posts.

But Republicans are generally favored to win back the Senate in November. If they do, they'll have new power to block presidential appointments, and Obama will have a new incentive to find ways to work around them. Minority Leader Mitch McConnell has also suggested he would change the rules back to allow filibusters on nominees. If all this happens, the Court's decision could be very influential in the last two years of Obama's presidency.