Meek Mill is a rapper born and raised from Philadelphia and Cassidy is from Brooklyn New York. All the beef between them had started with Meek Mill dissing Cassidy in one of his songs "Kendrick mar you are next". Cassidy wasnt fond of that and came back with a whole song dissing Meek Mill about how he isn't true and a fake rapper. This battle has been going on for a while and honestly its all just for publicity and what not. They really don't like eachother but it keeps the fan bases live and gives them what they want.