CPU Usage- It is the amount of time a CPU was used for processing the given instructions.
Memory Usage- It is the amount of RAM was used for processing the given instructions
Entry Processes- It is number of scripts are running at a single time. One entry process take fraction of a second to complete. So in 10 entry process you can have 10-30 visitors.
Number of Processes- It is number of process(script) can run in one time. One process take time till the execution of script.
I/O Usage- It is speed of data transfer between the hard disk and the RAM.