Your cPanel web hosting service is assigned x amount of CPU, RAM and Bandwidth resources. These resources are metered per second. It's a good idea to check through them every so often, as they can give you an idia of why your site is slow and if you need to optimise or upgrade to a bigger hosting plan.
NOTE: It's fine for resources to max out occasionally and have certain high spikes. These just suggest that a lot of users are visiting your site simultaneously or some periodic scripts which use many resources are being ran. You should only worry when your resource usage hits its limit consistenly for long periods of time, as this can result in your site slowing down heavily or becoming inaccessible.
To view these resources, log in to cPanel (click here for a guide) and click on the CPU and Concurrent Connection Usage icon. Note that on some older cPanel themes the icon is named Resource Usage.
The main resources you should look at/worry about are:
CPU Usage - The CPU is like the engine powering your hosting service. Everything your site does (show a page, read/write to the database, run PHP files, etc) will use a small amount of CPU.
Physical Memory Usage - Memory is mainly used by applications and PHP files on your website. As most web builders (WordPress, Joomla, etc) are just multiple PHP files working together, this statistic is important.
Input/Output Usage - This specifically refers to how much data you can read/write to your disk per second. Things like moving files around, writing to the database, generating images and making backups from WordPress or other CMS can increase this.
Entry Processes - Each PHP file that's ran on your site uses 1 Entry process. These run for a fraction of a second, so if the limit is 10 Entry Processes and each runs for 0.1 seconds, there won't be any issue with 10 people on your site at the same second.