Jummah Mubarak is an Islamic greeting used to wish someone a blessed Friday. It is a common phrase exchanged among Muslims on Fridays, especially before or after the Jummah (Friday) prayer. To respond to someone who says "Jummah Mubarak," you can reply with "Jummah Mubarak to you too" or "May you have a blessed Friday as well." It's a simple way to acknowledge the greeting and reciprocate the well wishes.