Your current inbox has a limited amount of space (1GB). This may be enough for most people but for those who send and receive many emails throughout the day it may quickly get used up. If you fill your space you will no longer be able to send or receive emails, so it’s important to keep on top of this.
The best way to avoid filling your inbox is to delete any unwanted emails from your inbox and sent items and be sure to empty your trash and junk email folders regularly.
If you only access your emails from one device, then another alternative would be to change your email account settings from IMAP to POP3 (the actual settings are identical) and set this to ‘not leave copies of emails on the server‘. The benefit of this is that you will then only keep copies of emails locally and so will not run out of server space. The downside is that you will only be able to see the emails on this one device, they will not be available to anyone else, and you must ensure that you back them up. These will then take up space on your device, which may or may not be a problem depending on its own storage capabilities.