It is a celebration of the coming to earth of Maitreya the Buddha of the Future. It is when Buddhists prepare and celebrate the return of the next Buddha known as the Maitreya. According to Buddhist scriptures Maitreya will be the successor to the current Buddha, Gautama Buddha who himself followed on from Siddhartha Gautama. Buddhists do not believe Buddha is a god.

They believe he was a human being who woke up (attained enlightenment) and can see the truth. They believe he regularly reincarnates to keep reminding us of what we as humans so easily forget whilst we live in this world. 

Although it is the last two buddhas and the Maitreya who feature in Buddhism, according to Theravāda Buddhism there were 27 earlier Buddhas.

Further Information

See https://en.wikipedia.org/wiki/List_of_the_named_Buddhas