The act is called the Voting Rights Act of 1965.
This act, signed into law by Lyndon B. Johnson, helped to remove legal barriers that African-Americans faced when it came to voting. For example, African-Americans were unfair targets of things like literacy tests, poll taxes, and the grandfather clause. All three of these barriers were put in place to limit the amount of African-American citizens voting in local, state, and national elections.
This law ensured that African-Americans received the same protection guaranteed to them under the 15th amendment.