The inode is the single file, which belongs to one user (cPanel account). It can be any file on your hosting plan, including emails, website files, databases. Inodes are counted automatically and results are displayed at cPanel panel.
Limit for shared hosting is 100000 and it’s more than enough for most websites. Most customers won’t notice this limit, because it’s way below what website needs, eg. default WordPress uses less than 1700 inodes.