Brexit: What You Need to Know

— -- President Barack Obama today waded into the raucous British debate over whether the United Kingdom should leave the European Union, saying such a move would be a mistake.

Writing in The Telegraph, Obama argued that Europe and the UK will be better off if they remain united.

"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic," he wrote. "So the US and the world need your outsized influence to continue – including within Europe."

U.K. lawmakers who want Britain to exit the E.U., the so-called Brexit, have been worried Obama will speak out in favor of the “remain campaign” and his op-ed in the British publication is sure to anger them.

More than 100 British parliamentary members recently penned a letter to the U.S. ambassador in London asking Obama to stay out of the debate.

"This is a chance for the British people to choose the path of their country," the letter reads. "Interfering in our debate over national sovereignty would be an unfortunate milestone at the end of your term as President."

During is visit to Britain, Obama will be meeting with Queen Elizabeth, who just celebrated her 90th birthday, and will hold a news conference with Prime Minister David Cameron, who wants his country to remain in the E.U.

All eyes will be on how Obama addresses the Brexit.

What is Brexit?

The term is an abbreviation for what is known as “British exit,” and references the possibility that Britain will withdraw from the European Union.

The country is expected to hold a vote in late June on a referendum on its membership in the E.U. British voters will decide whether the U.K. should cut ties with the E.U., a political and economic alliance made up of 28 countries.

Supporters of leaving the E.U., mostly conservatives, in recent years have called for a vote on whether to remain in the alliance, arguing a referendum on the issue hasn’t been held since 1975. Amid this clamoring, Cameron said during the 2015 campaign that he would hold such a vote if he won the election.

What happens if the U.K. leaves the E.U.?

Advocates of Brexit say that being outside the E.U. would boost the British economy by placing fewer regulations on consumers, employers and the environment. It also would give the country more freedom to establish its own economic and political policies.

The E.U. is the largest British trade partner, and those who oppose Brexit argue it would result in higher costs for goods and services. The departure from the E.U. would mean Britain could no longer benefit from being part of a single economic market and the trade benefits it brings, an argument that deputy national security adviser Ben Rhodes made to reporters last week.

"We believe that the U.K. has benefited from the single market," he said. "That is good for the British economy and that, in turn, is good for the United States economy, because we benefit from that relationship with both the U.K., but also the European Union broadly."

Many Brexit campaigners also say leaving will lower immigration, a priority for some, because the U.K. will no longer be bound by E.U. policies and could tighten its laws.

Is the Brexit a big issue in the United States?

Not really.

It hasn’t been a big issue for members of Congress, a topic in the presidential campaign, and even the administration has sought to downplay it as an issue, while officials hope British voters will vote to stay a part of the E.U.

"Obviously people on the Foreign Relations Committee are more focused than others, but Congress-wide, there’s no focus," Sen. Bob Corker, R-Tennessee, who chairs the committee, told POLITICO earlier this year about the Brexit debate.