Can someone explain exactly what is the "Domain Bonus" in D & D 5e / Next?

I am reading the rules of 5e, and I continue to see how you can add skill bonuses to attack rolls with weapons in which you are competent and skills checks in skills in which you are competent, but I can not find anything about what this is bonus REALLY. How do you determine which is the real numerical bonus that you add to your roll? And when everything is used?