The Washington Post has reported that when Donald Trump met with the Russian Foreign Minister and Ambassador, he revealed information to them that was highly classified, and so sensitive that it has not even been revealed to our closest allies -- information that could expose a source within ISIS. And BuzzFeed News says that the blunder by Trump may even be worse that was reported in the Washington Post.
Did Trump release the classified information because he is a blowhard and braggart who simply can't control himself? Or was this some sort of payback to the Russians for their help in the 2016 election? At present, we can't really know. And that makes it even more necessary for an independent prosecutor or bipartisan commission be appointed to investigate Trump's ties to Russia, and their involvement in the 2016 election.
Either way, Trump made a grievous error. While that error may be technically legal, it could also be an impeachable offense (by violating his oath of office).
The Lawfare website has written a comprehensive article on this, and I highly recommend you read the whole thing. Below is just a tiny portion of that article: