If you use IMAP to read emails that means emails are stored on your server. Over time this will consume space and your server has sufficient storage for years of normal use however some users may have more email accounts or have more attachments.
When you create an email account on the server you define the email account name, assign it a password and Quota size which can be set to a default value like 50GB of storage or unlimited. Unlimited simply means there is no hard limit other than the total space allocated to your hosting or email package.
As that account accumulates more emails as you approach quota you will need to either increase quota or delete / download old emails or if your entire server package is out of space order more space.
Your cPanel has a Manage Email Disk Space option you can use to delete old emails or you can setup a computer based system like outlook or thunderbird and set it to permanently download emails that meet your date criteria so they are downloaded to your computer and free up space on the server.