Extra fragments (after you 5 star a ninja) can be refined for credits, which you c*e to trade in based on the rewards listed on the page. Not all ninjas will have the same credit amount per frag, eg: Guy will only give 2 credits per frag, while Hinata will give 10.

when u have 5* ninjas already , the left over or extra fragments you get for those ninjas it can be use for refinement
once u get 3000 frag points , you can exchange it for 10 iruka fragments for free + receive seal or summon scrolls

Hello. You can only refine excess fragments of 5* ninja to gain points. The rewards usually depend on how much points you turn in. You can find out the rewards by accessing refine fragment through the ninjas interface.

Does this mean I have to get 15000 fragments??? No matter what *on I press, I can't refine.

That means you have to get 15,000 points. As stated previously, different ninjas are worth different amount of points. Either 2, 5, or 10. The reason you can't refine is because you have no points. Start collected 5 star ninja fragments and refine them for more points.