Complex sentence: A normal read or write cycle refreshes a row of memory, but normal memory accesses cannot be relied on to hit all the rows within the necessary time, so an abbreviated cycle called a refresh cycle is used.
Split into simpler sentences: A normal read or write cycle refreshes a row of memory, but normal memory accesses cannot be relied on to hit all the rows within the necessary time, necessitating a separate refresh process. Rather than use the normal read cycle in the refresh process, to save time an abbreviated cycle called a refresh cycle is used.