(Image Credit: Jason DeCrow/AP Photo)

President Obama presided over a rare and significant session of the U.N. Security Council today as leaders of 15 nations voted to unanimously adopt a resolution aimed at stopping the flow of foreign terrorist fighters across the globe.

It was only the sixth time in the Security Council's 68-year history that member country heads of state convened for a session, and only the second time a U.S. president held the rotating chairmanship. In 2009, Obama led a meeting on nuclear nonproliferation.

The White House says the resolution is a critical cornerstone of the long-term, global effort to destroy Islamic extremist groups, like ISIS and al Qaeda. Here's a short explainer on what the resolution is and what it does:

The resolution requires all 193 United Nations member states to "prosecute and penalize" people who travel or attempt to travel abroad for terrorism training, or who help finance such efforts. It would require countries to deny entry to anyone they have "reasonable grounds" to believe could be supporting or participating in terror-related activities. It would also compel countries to share airline passenger information records and other personal details with international databases to help track and prevent movement of suspected foreign fighters.

The resolution singles out "foreign terrorist fighters" as those individuals who "travel to a State other than their States of residence or nationality for the purpose of the perpetration, planning, or preparation of, or participation in, terrorist acts or the providing or receiving of terrorist training, including in connection with armed conflict."

The National Counterterrorism Center says there are about 15,000 individuals from 80 countries fighting or training with Islamic extremists inside Syria and Iraq. The total includes an estimated 2,000 Europeans and over 100 Americans who "have attempted to go or have gone." The FBI says some U.S. fighters, who received training abroad and/or fought alongside rebel groups in Iraq or Syria, have returned to the United States and are under active monitoring. Meanwhile, western passport-holders still abroad are of great concern to homeland security authorities because they can enter the country without having to apply for a visa and undergo additional scrutiny.

Security Council resolutions adopted under Chapter 7 of the U.N. Charter are binding on all 193 member nations, but there is no "hard enforcement mechanism in place," U.S. officials concede. It's largely global peer pressure that will lead to wider implementation of the resolution as an international norm. One challenge facing enforcement is competing definitions of what a "foreign fighter" is: a terrorist in one country's eyes could be a freedom fighter in another's. Another challenge is tracking and cracking down on nation-less cyber-recruitment of violent extremists, while balancing civil liberties and Internet freedom at home. There's also not a global consensus on what constitutes a terrorist organization. Nonetheless, the White House says simply "galvanizing" other countries to be more attuned to the problem of foreign fighters is a key objective.

The resolution was unanimously adopted by members attending the meeting, which included French President Francois Hollande and U.K. Prime Minister David Cameron. Fifteen hands went up when Obama put the measure up for a vote. No one opposed.