Methods such as literacy tests and poll taxes were used to disenfranchise African American voters in the early 1900s, restricting the ability of African Americans to exercise voting rights.
Poll taxes means you have to pay a tax before allowed to vote. That's a way of preventing poor persons (like blacks in the South) from voting.
Literacy tests would require persons to pass certain standards of reading and writing in order to qualify for voting. These tests were aimed at blacks, who had not had access to the same education as whites.
The black community had less access to education than whites even after slavery was ended; thus their literacy rates were lower. They also experienced much poverty because of prejudice against them in the economic system of the country, so poll taxes could keep them from going to the polls to vote.
States also employed "grandfather clauses," which were exemptions granted by some states to those whose forefathers ("grandfathers") had full voting rights prior to the Civil War. That way, if there were poor or illiterate whites, they could vote freely while blacks (whose ancestors had been slaves) were subjected to the laws restricting their voting ability.
These sorts of restrictions against black voters prompted much of the activism of the civil rights movement that began in the middle of the 20th century.