From Encyclopedia Britannica:
statutory or constitutional device enacted by seven Southern states between 1895 and 1910 to deny suffrage to American blacks; it provided that those who had enjoyed the right to vote prior to 1866 or 1867, or their lineal descendants, would be exempt from educational, property, or tax requirements for voting. Because the former slaves had not been granted the franchise until the adoption of the Fifteenth Amendment in 1870, these clauses worked effectively to exclude blacks from the vote but assured the franchise to many impoverished and illiterate whites. In 1915 the Supreme Court declared the grandfather clause unconstitutional because it violated equal voting rights guaranteed by the Fifteenth Amendment.
Read the Grandfather Clause used by Louisiana right here.