An inode is a data structure used to store all the information about the files on your hosting account, which include emails, files, folders and everything you upload, create or store on the server.
The number of inodes indicates the number of files and folders you have. For example, shared and reseller servers only allow 150,000 files per cPanel account. This number is the "Soft Limit", i.e. you will still be able to upload files once the limit is reached. However, Exabytes will send you a notification to urge you to clear the files and folders.
As for a hosting account, the limit is currently 250,000. If your account exceeds 250,000 files, a notification email will be sent to you and you will be required to clean the excessive inode number within one week. Although exceeding the limit is in violation of our Terms of Service and can result in possible suspension, shared hosting account holders should not be too worried as most shared hosting accounts inode usage is still way below the limit.
VPS accounts are limited to 1,000,000 inodes. In other words, you will be unable to upload files to your account once the threshold is reached.
Dedicated servers however, have no inode limitation. Its total inode usage depends on the file system of the server.
To reduce the inodecounts, you only need to delete the files or emails you no longer need. Alternatively, you could also download the emails to your local system.
Lastly, TOS Inode violations also include (however not limited to) continuously publishing and deleting large number of files (i.e. 10,000+) without pre-approval.