Metempsychosis is the belief that the soul can migrate from the body after death to inhabit another body: reincarnation, in other words. As Sonya goes on to say, the Ancient Egyptians believed in the transmigration of the soul after death. Evidence for this can be found in their many Books of the Dead. They later came to believe that souls could be reborn in new generations as a way of explaining family likenesses.
The idea of reincarnation also occurred in Ancient Greek thought, possibly as a result of contact with Egypt. Today, the most widespread belief in reincarnation occurs in Hinduism, where it is believed that the soul will be reincarnated as a higher or more lowly species depending upon its behaviour during its current life.
Mummers were troupes of folk actors who performed allegorical plays at certain times in the year, such as Christmas and Easter, and various Saint's Days. The tradition started in England, but swiftly moved to other countries. Mummers, drawn from the local community, would perform in houses or inns; they would be heavily costumed and disguised.
This can be read as a reference to Nikolai Gogol's novel Dead Souls, published in 1842. In a famous passage in this novel Gogol compares Russia to a troika: "And you, Russia of mine--are not you also speeding like a troika which nought can overtake? Is not the road smoking beneath your wheels, and the bridges thundering as you cross them, and everything being left in the rear, and the spectators, struck with the portent, halting to wonder whether you be not a thunderbolt launched from heaven?"
A troika is a Russian harnessing system used to pull a carriage or sleigh with three horses side by side.