what does 5-2-2-2.5 mean

when peopel say the RAM is 5-5-2-2.5 what does that mean??

It means the memory is running with,

a CAS Latency (tCL) of 2.5,
a CAS-to-RAS Delay (tRCD) of 2,
a RAS-Precharge (tRP) of 2,
and a RAS-Pulse-Width (tRAS) of 5.

An explanation of memory timings can be found here .

Another good resource about timings:
http://www.lostcircuits.com/advice/bios2/6a.shtml
it goes on for a couple pages.