Reconstruction was the period from 1865 - 1877 where the government of the United States tried to put the country together again after the Civil War. In general, historians consider it an unsuccessful effort for several reasons.
Politically, the government did convince southern states to rejoin the Union in a fairly simple process. They also managed to pass the 13th, 14th and 15th amendment. Past that, not much went well.
The Freedmen's Bureau was underfunded and cut short, leaving the vast majority of free slaves uneducated and still in the South. There was no land reform, meaning slaves were forced into a sharecropping system and did not own their own farms, which might have made them more independent, equal and successful.
The Black Codes and other laws restricting former slaves, though clearly unconstitutional, were not challenged in court or struck down by local military authorities, leaving African-Americans virtually unprotected and subject once again to working for whites involuntarily.
And finally, the effort of Reconstruction was cut off after only 12 years, leaving the economy of the South still in ruins and its population largely in poverty.