Poll taxes required citizens to pay a fee to register to vote. These fees kept many poor African Americans, as well as poor whites, from voting. The poll tax receipt displayed here is from Alabama.
Smithsonian National Museum of American History’s exhibit Separate is Not Equal: Brown v. Board of Education, 2004, specifically from the Segregated America section:
"Poll Tax Receipt," in Teaching with Laurel Grove School, Item #84, http://chnm.gmu.edu/laurelgrove/items/show/84 (accessed July 28, 2014).