Sortition means random selection, or the act of casting lots. Sortition is the approach used to randomly select people for a citizen’s assembly, parliament or congress
For example, to be in a citizens’ assembly you must be randomly selected to take part. You cannot buy your way in, nor can you simply turn up. This makes it fair for everyone. Everyone should, ideally, have an equal chance of being selected. Citizens’ assemblies are an innovative and powerful way to make political decisions. They break the hold of career politicians on decisions, and bypass the powerful vested interests that often exert undue influence on policy outcomes. They are a way to put representative groups of people, brought together in informed and deliberative environments at the centre of decision making.
Sortition was the method used for choosing public officials in some ancient Greek city-states. It was used especially in the Athenian democracy, from which most information about the practice is derived. With few exceptions, all magistrates were chosen by lot, beginning with the archons in 487–486 bc 
 The Sortition Foundation, https://www.sortitionfoundation.org/why