"Wa Jazakillahu Khairan" is an Arabic phrase meaning "May Allah reward you with goodness." It's a common expression used by Muslims to express gratitude to someone who has done them a favor or provided assistance. The appropriate response to "Wa Jazakillahu Khairan" is "Wa Antum Fajazakumullahu Khairan," which means "And may Allah reward you as well with goodness." This exchange of blessings strengthens the bond of brotherhood and sisterhood within the Muslim community.