An inode is a data structure which contains information about a regular file, folder such as its size and owner. The number of inodes you use represents the number of files/folders you have on your web hosting account. Many web hosting companies restrict the inode usage on the shared hosting server. Hence, even if they offer unlimited disk space, you cannot use it because of inode limit.
When a file is opened, the file’s inode is read by the kernel. If you have more files/folder in your account, the more inodes you use. Generally one file uses one inode.
As shared hosting customers, you may receive the notification regarding inode usage from your hosting provider. Generally the web site contents do not require more inodes. In most cases, catch-all account consumes more inode. You can delete/download emails from your catch-all email account to reduce the inode usage and if you do not require catch-all account, it is advisable to disable it. Also, if your account has many email accounts, it is advisable to download the emails via email client software viz. Outlook, Thunderbird etc.