It is term used by Muslims, to greet each other during festivals. The term translates into English as, 'The Blessed Festival'
Eid means the festival itself, Mubarak means Blessed. It is a social and cultural aspect that stems from the deep roots of Islam.
However people also have different other ways to greet each other on Eid-al-Adha/Bakrid and Eid-ul-Fitr.
People in Arabia, may in addition say, kul 'am wantum bikhair, meaning, (May) you be well every year"
It is also said that people who were with Prophet Muhammad (Peace be upon him), greeted each other on Eid-ul-Fitr as, Taqabbalallhu minn wa minkum, meaning, (May) God accept from us and from you, (our fasts and deeds)
In Turkey, where Eid Mubarak is not very common, people greet each other, saying, 'Bayramınız mÜbarek olsun' or "Bayramınız kutlu olsun", meaning, "May your holiday be blessed"
Whereas in Afghanistan, people greet each other with the phrase, Akhtar de nekmregha sha, meaning, may your festival be blessed.
Hence Eid-Mubarak (What is it?), simply means the Muslim way of greeting each other during festivals.