I get to read some numbers for average server load when i type 'w' at the command prompt of my server.
what do those numbers mean and how are they calculated ,and what can be inferred from them ?

Please help me know these thing.

They are the load averages for the past 1, 5 and 15 minutes repsectively. "load" lets you know how much is in the CPU run queue (a/k/a runq).

On Linux it is not unusual to see high load averages depending on what you're doing (e.g. an Oracle Database server). The idea is to figure out what the load average is normally (i.e. observe your system for a few days after install to figure out what is normal) then address issues when you see load averages that are outside the norm.

I've seen a system running a load average of over 30 that continued to run other processes. However seeing it go up to 30 let me know to investigate what changed as its normal load average hovers around 6. By investigating I found there were hung processes that couldn't complete due to a locked file. Because they couldn't complete they stayed in the run queue and each subsequent process that attempted to access the same locked file added to the run queue.

This is to say on Linux I've found that load averages in and of themselves aren't as useful an item to track as they are on UNIX variants I've worked with.