White terror was comprised primarily of groups like the Knights of the Ku Klux Klan and the White Camellia. Both terrorized Blacks who attempted to vote or participate in government in any form. Most members of the Klan were influential whites who wore hoods and masks to disguise themselves. Many were ministers, magistrates, lawyers, even teachers.
Rather than deal with the terror spread by these groups, Republicans were more intent on retaining control of the White House. When the election of 1877 was contested, they agreed to end Reconstruction--or more appropriately abandon it--in exchange for the certification of Rutherford B. Hayes as the winner of the Presidential election. Most scholars consider Reconstruction to have been a failure; as Blacks were no better off at its end than at its beginning; other than they were now freedmen rather than slaves.
Although terroristic tactics were not the only thing that helped to end Reconstruction, they certainly had an impact. Some whites in the South used terrorism to disrupt the Republican Party and to intimidate blacks so they would not vote for the Republicans. These actions helped to bring about "Redeemer" governments in many Southern states even before Reconstruction formally ended in 1877.
Various groups used violence and threats of violence to break up Republican meetings. They committed acts of violence against black people who tried to vote or who tried to stand up against them. By doing these sorts of things, these groups drastically reduced the ability of the Republican Party to win elections. When this happened, "Redeemer" governments won elections and started to dismantle Reconstruction in their states.