In the presidential election of 1876, neither Rutherford B. Hayes (the Republican) nor Samuel Tilden (the Democrat) had enough electoral votes to win the election outright. This was because the votes of three southern states were disputed. Those states each sent two sets of electors to the electoral college -- one for each candidate.
An electoral commission that was set up to solve this problem ended up ruling that the Republicans had won all three of the states whose votes were disputed. Many of the Democrats felt that there must have been some sort of deal made that induced Democratic members of the commission to allow this result. This is why it is sometimes called the Corrupt Bargain.