Your cPanel hosting service has a number of resources that you should monitor regularly to make sure your website is consistently stable.
To view your resource usage, first, in to cPanel (click here for a guide). You can glance quickly at the table on the left or right and get a quick overview of the resource usage. However there are a number of resources you should monitor closely and make sure they aren't exceeded.
CPP/Memory/IO usage - This is the physical computer resources allocated to a cPanel account. They aren't metered, but they do have limits that are calculated based on how much of a resource is used each second. While it's no problem to have these resources hit their max occasionally, if they do this regularly then your site will be inaccessible until the resource use drops. Click here for a guide explaining these resoruces in depth.
Disk Space: Your disk space is the total size of the files of all data on the cPanel account. If this limit is hit, then you won't be able to add any more files. Actions like sending/receiving emails, writing to the database and making changes to your website all add to the disk space. This means that emails and certain website functions may stop if your disk space hits it's limit. Click here for a guide on using the Disk Usage tool to break down your disk space and find where data is stored.
Email Disk Space: This is related to the Disk Space resource above, but also gets it's own statistics. Each email account has a limit on the total size of all emails in it (including sent, draft and custom folders). If an email account reaches 100% disk space usage, you won't be able to send or receive emails from that account until the limit is increased or emails are deleted to reduce space. Note that setting an email account's quota to unlimited just sets the limit to part of the total disk space limit in cPanel. Click here for a guide on using the Email Disk Usage tool in cPanel, which provides a simple method of managing this resource.
File Usage: Similar to Disk Space, most cPanel accounts have a limit to the total number of files there. This includes all emails, website files and system files. Click here for a guide which explains this resource in depth.