The grandfather clauses were laws that allowed people to vote if their fathers and grandfathers had voted. They were made because the poll taxes and the literacy tests prevented some whites from voting, and by making the grandfather clauses, they were able to vote.
I was looking for the answer but I couldn't find one that was very convincing, so I decided to add my own. Hope this helps some people in the same situation as me haha. ;P
btw: I did get this answer from the textbook Discovering our past A History Of The United States Mc Graw Hill Education or whatever.