Mar 10:31 But many [that are] first shall be last; and the last first.
Alternative: But the great, foremost shall be the lowest and the lowest the foremost.
The context of this chapter is about human relationships. It starts discussing the exclusivity of marriage and importance of children. It them moves to a discussion about possessions and how the possession of things can be a handicap to our spirits, but the possessions of relationships is not. On the contrary, the spirit gets us more relationships despite the rejection of society.
This idea continues here. While the normal English translation emphasizes the use of these words in terms of order, in the context of relationships, Christ is talking about social order and who we see as the highest and the lowest. Those with many personal, caring relationships are rich not matter how physically costly such relationships are. Having children may be costly, but they provide the wealth that counts.
"Many" is from polus (polys)," "great (in size or power or worth)," and "large (of space)." As an adverb is means "far," "very much," "a great way," and "long."
"First" is from prôtos (protos). In place, this means "the foremost." Of time, it means "the initial." In order, it means "the first." In math, it means the prime numbers. Of rank or degree, it means "the highest" or "the best." This was the word used to mean "the first" in the parable of the landowner hiring workers.
"Shall be" is from eimi (eimi), which means "to be," "to exist," "to be the case," and "is possible." (The form used here is the future form esomai.)
"Last" is from eschatos (eschatos). In space, this means "furthest." In degree, it means "uttermost" and "highest." In persons, it means "lowest" and "meanest." Of time, it means "last" and "ending."