Oveview actions are used to display items matching a field with the selected (if it is selected from the view) or currently open (if you have it open) item.
To make it more clear. Suppose you have a list of open service calls and Assigned;To Person is a part of the list. Now let's say that you want to see what other service calls are assigned to this person and they are not closed. You can used advanced find or use an overview action.
Let's make one named "All open calls assigned to Assignee"
Go to Overview Actions and select Service Call. Right click and select "New Overview Action"
Give it a brief description (Text) and optionaly add a comment.
Then Select from the Item "Service Call" and from the view select a view that displays service calls which are not completed or closed. If you don't have a view like this, press Add to make one.
On the left pane, select from Condition "Assignment;To Person" As operator select "equals" and as second item select again "Assignment;To Person"
Press Add to list and then OK. The action is ready. Now select an open service call that is assigned to person. From the action select the newly constructed Overview Action. A new window will popup displaying currently assigned and not completed or closed service calls to this person.
The better part is that you can use it to UI Rules. If you are using dispatchers to assign the ticket this could make their life easier.
Create a ui rule that will be fired when an item is changed. Check for Assignment;To Person is not empty. As action select "Overview Actions" to execute the action constructed above.
Now, each time an item is assigned to this person a list will popup informing the assignor the current open items assigned to assignee.