What limits mean.
CPU
Processing capacity. Consumed by generating pages, running queries and processing tasks.
PHYSICAL MEMORY
The memory your account's processes can use at once.
I/O
The rate at which your account reads and writes to disk.
ENTRY PROCESSES
Simultaneous requests being handled.
Not simultaneous visitors, though the two are related.
PROCESSES
The total number of running processes for your account.
INODES
The count of files and folders.
Email messages and cache files each count as one, which is how accounts exhaust inodes while disk space looks fine.
THROTTLING
Slowing an account that exceeds a limit, rather than stopping it.
SUSPENSION
Disabling an account, usually for non-payment or abuse.
RESOURCE USAGE
The cPanel section showing your consumption against each limit.
WHAT TO CHECK WHEN A SITE IS SLOW
That section, before anything else.