I don't completely understand the film. Could someone explain it to me.

Your questions are too broad for me even to understand well what you want explained, but if you ask some more specific questions, I'm sure people here will be glad to answer them. I don't think you have to be either American or religious to understand the movie or the play, but I suppose it might help to know a little about both America and some religions, especially Mormonism and Judaism, to enjoy it. However, I think I can safely (if broadly) say that there isn't "some American core tradition" that you missed and need to know about to understand the play or the movie.

It has nothing to do with any American core tradition. The play and film is about so many things. For me, it was about AIDS, loss of love, repression of one's sexuality, religion, being afraid(and fleeing like a coward)because of seeing the one you love deteriorate, its about the Reagan era American ideas and the disillusion of those ideas.Thats what I got from the series, of course I'm no expert. There is much more but I'm not the type to type paper thesis long posts. You don't have to be American to understand or enjoy the film.

The "Angels" reference in the title is camp: that is to say, it is self-mocking and ironic. The word itself is not used in a sacred sense, but in the sense that you might say "You angel" when someone made you a cup of tea.

In the 1970s, the behaviour of male homosexuals was extremely outrageous. Major misbehaviour in private bath-houses by hordes of drug-deranged men caused revulsion in normal society.

When AIDS happened, there was a widespread feeling that male homosexuals had brought the disease on themselves with their outrages. This prejudice caused delays with the preparation of a remedy that was already going to be very difficult because of the disease's effect on the auto-immune system.

This film wasn't about gay guys who get AIDS and die but rather heterosexual men who aren't gay but like to play around with boys. You can look like a duck, and smell like a duck, but you aren't a duck.

I don't get the angels in the film - i fast-forward through those scenes because the dialogue is too contrived and goes nowhere.

First of all this play.. (or film) takes us back through tradition, to something strictly and culturally fundamentally American, i.e. Mormonism. Thus, we also have a conflict with the past throughout the film.
What is America sybolizes? What is the driving force of America (or any other people really)? It is progress, the need to progress no matter what. However with this progress, there is something missing, and it is missing some of its citizens in the midst of this corrupt political and media-social propaganda situation. So... back in the 50 the word homosexual was not even allowed to be played on the television, as a homosexual you were sidelined, which is why Roy Cohn cannot accept that, because then he will have no "clout"... with the AIDS epidemic or the pestilence of AIDS what we have is the inability for the mainstream culture to ignore the minority and to put them on the sidelines anymore, they become important and then as a result they must be granted the rights just as any other, thus they become Angels for that reasons, as Prior says that they are all prophets.

The thread running through the film about God's abandonment goes to the central question of the existence of evil and suffering in the world and religions inability to explain , or relieve it. This can be summed up by the quotation from the ancient Greek philosopher, Epicurus:

“Is God willing to prevent evil, but not able? Then he is not omnipotent.
Is he able, but not willing? Then he is malevolent.
Is he both able and willing? Then whence cometh evil?
Is he neither able nor willing? Then why call him God?”

But you ARE Blanche ... and I AM.