Hi Sarif,
You have correct understanding about SAP UI5 and ABAP i.e. UI5 used to create User Interface for you while ABAP will use for creating Backend services for your interface.
Now coming to your question, "as an ABAP developer where do you fit in in the scenario?"
So the answer is you may fit for creating OData Services to make backend ready for the UI.
because one can create attractive UI but its incomplete until it interacts with your server and that interaction can be possible with the help of SAP Gateway by creation of CRUDQ (Create, Read, Update, Delete, Query) operation. You may refer this link to know more in detail about CRUDQ creation : Let’s code CRUDQ and Function Import operations in OData service!
Talking about Fiori then all the applications provided in Fiori catalogs has already created relative OData services, you just need to install those product version and configure the services, also you need to provide specific authorizations to your user.
So thats it, by this way an ABAPer can play his role efficiently in SAP UI5 orchestra.
Regards,
Chetan.