Complex sentence: It means, the sum of Z and a larger part of U gets added, at the end of each production period, to the owned-money component of the total stock of money; its other component being the amount of money borrowed from commercial banks.
Split into simpler sentences: It means, the Z and a larger part of U together gets added at the end of each production period to the owned-money component of the total stock of money. The other component of the total stock of money is the amount of money borrowed from commercial banks.