Original: SummerSlam became one of World Wrestling Federation (and later World Wrestling Entertainment)'s most successful events and one of the "Big Four" pay-per-views, along with WrestleMania, Survivor Series, and the Royal Rumble.
Simplified: Along with WrestleMania, Royal Rumble and Survivor Series, SummerSlam is one of the "Big Four" pay-per-views and was one of the original four yearly WWE pay-per-views.