what does apache (busy workers , idle workers) means?

If you have only very few or none idle workers it means Apache is using all the processes it is allowed to use and new incoming requests have to wait for older requests to finish before they can be handled. In this case, increasing the maximum allowed processes in your configuration file might help with performance under certain circumstances.

This is true if your system has reserves to handle additional apache processes. Each of them will need memory, CPU time and disk I/O.

If you don't have these reserves, increasing the MaxClients might be even contraproductive, as more processes will have to "fight" for the resources and it might be that reducing the number of maximum clients will improve performance if you already hit some bottleneck.