The US Secretary of State is that country's chief diplomat and foreign relations expert. He/she is head of the State Department, which looks after matters relating to the State, this is comparable to Britain's "foreign secretary".
The first Secretary of State was Thomas Jefferson under President George Washington. The current Secretary of State is Mike Pompeo, replacing Rex Tillerson, appointed by Donald Trump. Obama's second term saw John Kerry in office. Hillary Clinton served during Obama's first term, from 2009 until 2013. Condoleezza Rice served in this role during the second George W. Bush term, from 2005 until 2009.