In order to prevent African Americans from exercising their right to vote.
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.
In addition to poll taxes, other measures were also used to block blacks from participating in the voting process, such as literacy tests. 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.