Poll taxes and literacy tests were established to limit the voting rights of African Americans.
Following the freeing of southern slaves in Abraham Lincoln's "Gettysburg Address", which led to the freeing of slaves in southern-occupied territories, and the slaves in the border states that were allied to the Union. This soon led to the defeat of the Confederates that surrendered in the Appomattox Court, where the South surrendered to the North. This soon led to the freeing of slaves, and the Reconstruction Age, in which Northern abolitionists then pushed to give equal rights for the African Americans. In the effort to stop these rights, the South tried to find different ways to limit their previous slave's rights, including Poll taxes, literacy tests (which limited their political voting powers), producing groups such as the KKK (Ku Klux Klan), which was a terror group that lynched and killed African Americans, and separation of White and African American public rooms. All of these (& more!) were used to limit the rights and make life harder for the African Americans.
Specifically for this question, poll taxes were instituted against the African Americans, because following the outcome of the Civil War, many African Americans could did not have any money. In an effort to limit both the Republic Party and their supporters, the South instituted the Poll Tax, which forced all voters to pay a certain fee to cast in their votes. However, this also limited the ability of poorer southerners, and so they used literacy tests instead. With the use of literacy tests, the probability of Southern whites being left out was very small while, as the percentage of previous-slave population was being able to read and write was very small), it led a greater and concentrated discrimination against the African Americans.
While literacy tests were effective in their beginning years, they waned after many African Americans started to attend and learn how to read and write. This soon led to an increase role of the KKK to keep African Americans from voting.
In the end, all of these were just a way for Southern whites to keep African Americans from voting and voicing their rights, and essentially failed, as we can see today.
