Hardware Secrets learns you in detail what numbers like 7-7-7-21, 8-8-8-24, or 9-9-9-24 after the memory specification mean. You can read it over here.
"DDR, DDR2, and DDR3 memories are classified according to the maximum speed at which they can work, as well as their timings. Timings are numbers such as 3-4-4-8, 5-5-5-15, 7-7-7-21, or 9-9-9-24, the lower the better. In this tutorial, we will explain exactly what each one of these numbers mean."