To use this, you map from a data layer variable which contains one of several possible trigger values, and for each value you type the value in the input field called "trigger" and you select the appropriate event from the dropdown.
In the example screenshot below, you'll see 3 different trigger values mapped. Tealium concatenates the two fields for each. So, for example, the first one has a trigger value of "productClick", and a dropdown selection of "product_click". And the second one has a trigger value of "productRemove" and a dropdown selection of "remove". In each case, the field containing the trigger value is "event_type" in my example (but can be any field you want.
Of course, you need to also to put the trigger values into the data layer variable. So, you might do this directly in your application (when the event occurs) or else through an extension. Either way, you're just putting a value into the variable whcih later you'll use in the mapping as the "trigger" for one of several events. In the example below, the value in the data layer variable "event_type" has been populated by the application whenever the event occurs (in a utag.link function call).
And finally, you'll want to make sure you're using the e-commerce extension (or else overriding all the standard mapping for e-commerce variables) so that the related data is also set. And in your utag_link calls (or in your extension) you need to supply the other e-commerce variable values like product ID, quantity, etc.