This functionality allows users to add a specific comment that throws the selected event. You can use that for a number of various automatizations such as auto-transitioning and issue updates.
In the interface, it adds the custom reply button in the comments:
Navigate to Jira Administration → Manage Apps → Teamlead Toolkit.
Select an event to be thrown (you may need to add a custom event beforehand), specify a name for the button and tick the 'Use custom comment' checkbox.