Nationalism, the way Trump uses it, really has only two components:
1) As individuals, we should feel greater loyalty to, and compassion for, our fellow Americans, relative to the citizens of any other nation. This does not mean we should not feel compassion for the worlds suffering, but rather that we recognize there remains plenty of work to do here.
2) On a national, policy level, when there is a conflict between American interests and those of some other nation, we side with America. This should be self evidently the JOB of our government, but Obama, particularly, pursued a different policy, one which saw our national leader abasing himself in front of two bit dictators like Fidel Castro.