UPDATE: Fox News sent out a tweet quoting National Security Advisor H.R. McMaster, who also attended the meeting between President Trump and the Russian Foreign Minister. According to McMaster: "The story that came out tonight, as reported, is false."
According to The Washington Post, during his meeting on May 10th with Russian Foreign Minister Sergey Lavrov and Ambassador Sergey Kislyak, President Trump revealed "highly classified" material:
The information the president relayed had been provided by a U.S. partner through an intelligence-sharing arrangement considered so sensitive that details have been withheld from allies and tightly restricted even within the U.S. government, officials said.
The partner had not given the United States permission to share the material with Russia, and officials said Trump’s decision to do so endangers cooperation from an ally that has access to the inner workings of the Islamic State. After Trump’s meeting, senior White House officials took steps to contain the damage, placing calls to the CIA and the National Security Agency.
The Post quotes an anonymous U.S. official "familiar with the matter," who said what the president revealed was "code-word information," meaning it was incredibly sensitive, highly-classified material. The official added that Trump "revealed more information to the Russian ambassador than we have shared with our own allies."
Once again, according to "an official with knowledge of the exchange," Trump was allegedly bragging about the quality of the intel he gets. He then revealed information regarding an Islamic State plot, allegedly dealing with the possibility of explosive laptops on airplanes. He also revealed the city in which the intel was discovered.
The identification of the location was seen as particularly problematic, officials said, because Russia could use that detail to help identify the U.S. ally or intelligence capability involved. Officials said the capability could be useful for other purposes, possibly providing intelligence on Russia’s presence in Syria. Moscow would be keenly interested in identifying that source and perhaps disrupting it.
It's important to understand that this is new information coming from anonymous sources. It may be accurate, or it may be misleading. Further reports will be necessary to corroborate what The Washington Post is claiming.
However, a note of caution for any Trump supporters who may be quick to dismiss this story outright — if a news organization made the same allegations against former President Obama that The Washington Post is making against Trump, would it be so easy to dismiss?
Unless and until more information comes to light, this story remains unsupported.