What is a literacy requirement for voting and how was it used to deny suffrage to certain groups

Between 1890s and 1960s, everyone who wants to vote must take a literacy test. 

Literacy test is a test wherein they can assess your ability to read and write. This was used to deny suffrage to certain groups particularly to African Americans. 

Majority of African Americans were slaves and they were not given the opportunity to attend school and be educated. Thus, they were illiterate or not able to read nor write.

Whites who were illiterate can still vote when they can meet the grandfather clause, a clause that states that they can vote despite being illiterate as long as they can prove that one of their ancestors was able to vote before 1867.