In 2000, the Presidential election was between the Republican George W. Bush and Democrat Al Gore.
When the election finished, George W. Bush had very narrowly lost the popular vote of the people. However, the Electoral College actually makes the final decisions on the Presidential elections.
Those who are a part of the Electoral College are somewhat expected to vote with those who they "oversee" and essentially vote for. But they do not have to, and such is not always the case.
So in 2000, after George W. Bush had lost the popular vote in the election, the Electoral College decided that they were going to vote in George W. Bush, even though Al Gore had won the popular vote of the people. This is the controversy regarding the Electoral College.