Eid Mubarak or (Arabic: عيد مبارك) is an Arabic term that means “Blessed Feast/festival”.
If someone says Eid Mubarak to you, it is polite to respond by saying 'Khair Mubarak', which wishes good will on the person who greeted you. You could also say 'JazakAllah Khair' which means thank you, but literally translates as 'May Allah reward you with goodness'