What does the Invoke Custom Function action in Workflows do?

The Workflows module provides you a simple interface to add custom actions that execute on saving an entity.
Vtiger has a Custom Task function of entity method tasks that calls certain methods on Vtiger entity objects. Custom Tasks provides a way for developers to add custom actions to workflows. Let us walk through creating a Custom Task.

For example, if you want to compute natural Math Log as a custom operation, then first we need to register a method that will perform the operation. The method needs an operation name, file path (can be anywhere, even in a custom module), and a function name. 

Let’s say you have a custom module called Deals. You can register a method in the postinstall event(vtlib_handler API of Deals.php file) of the module. The event is triggered after the module is installed. For more details on how to write a module, click here.
To learn more about the action, click here.