It's all in terms of 'clocks' which are basically references to nanoseconds in time. There's a LOT that can be said about RAM, so your best bet is to google up some information. Latency basically is referring to how long something takes to get from one place another. So if it takes you 10 minutes to get to the store and lets suppose 10 minutes is 1 clock then to get home would be 20 minutes, or 2 clocks...thus your 'latency' from getting to the store and back home would be 2.
That's a really rough and probably not that great of an example though lol
Go there and scroll almost to the bottom where you see giant black text that says DRAM TIMINGS: with some brief information on timings.