The emails were received by Hillary Clinton when she was secretary of state and were subsequently stored on her private email server. The documents on Clinton's server have been released due to rulings from U.S. courts.
One email from then-Secretary of State Colin Powell to former President George W. Bush, details an upcoming meeting between the President and British Prime Minister Tony Blair which was to take place from April 5-7th, in 2002. This meeting became known as the Crawford summit and was widely speculated to have been used to discuss the invasion of Iraq. The release of these emails shows that this was, in fact, the case.
Secretary Powell Writes:
Blair continues to stand by you and the U.S. as we move forward on the war on terrorism and on Iraq. He will present to you the strategic, tactical and public affairs lines that he believes will strengthen global support for our common cause