Every hosting plan comes with a certain amount of disk space (how much data - files, emails, databases - you can store) and sometimes a bandwidth allowance (how much data can be transferred when people visit your site).
To check your usage, log into cPanel and look at the statistics panel, usually shown on the left-hand side or the main dashboard - it displays your current disk space and bandwidth usage against your plan's limit.
If you're getting close to your limit, common culprits are large email attachments left on the server, old backup files, or a growing media library on your website. Clearing out anything you no longer need, or upgrading to a bigger plan, are both quick fixes - just reach out to your hosting provider if you're not sure which option makes sense for you.