Original: For this reason, a read miss in a write-back cache (which requires a block to be replaced by another) will often require two memory accesses to service: one to retrieve the needed datum, and one to write replaced data from the cache to the store.
Simplified: A miss in a write-back cache (which requires a block to be replaced by another) will often need two memory accesses: one to get the needed datum, and another to write replaced data from the cache to the store.