What was the grandfather clause? why was it created? why do you think these voters were called the pre-1867 voters?

Grandfather clause, statutory or constitutional device enacted by seven Southern states between 1895 and 1910 to deny suffrage to African Americans. 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

The grandfather clause was a statute enacted by American southerners after the reconstruction that allowed potential white voters  to circumvent literary tests, poll taxes, and other tactics that hindered southern black people from voting.

they were probably called pre-1867 voters because the 14th amendment was passed in 1886, granting black people citizenship in America.