API functions allow you to view and change account data without having to log in to the cPanel interface.
(You can issue API tokens to allow others to perform API functions with your account data)
Example: You could issue an API token to a reseller. The reseller could use this token to check disk usage.
How to create an API Token in Cpanel
1- When accessing your cPanel, go to the Security tab and locate the item "Manage API Tokens"