This fight happens in Chapter 6. It is caused by the fact that Stradlater is implying that he had sex with Jane Gallagher, or at least went and made out with her in the coach's car. The fight tells us a couple of things, in my opinion.
First, it tells us that Holden has this old fashioned view of women. He thinks that it is his place to try to fight someone who has been making out with a girl he likes. He is either being chivalrous (nice interpretation) or very possessive -- implying she belongs to him.
Second, it tells us that Holden is really immature. He keeps calling Stradlater a moron. He says Stradlater is a moron because he doesn't want to discuss things. But it's not clear why Stradlater should talk about what exactly he did in the car with Jane. At any rate, Holden just keeps calling him a moron over and over, which seems pretty immature to me.
In the novel "Catcher In The Rye" by J D Salinger, the author shows the intense dislike that Holden Caulfield shows towards fellow school mate Stradlater. Holden seems to be of the opinion that Stradlater is a "phony" - now he thinks he is a "moron" as well.
Caulfield seems to think that Stradlater (and all the other rich boys except himself) are part of a fake establishment structure which he wants no part of. He thinks that in reality Stradlater is a "secret slob." He feels superior. He has the typical adolescent difficulty in expressing himself,only worse. S when he finds out about Stradlater's episode with "his" girl, he cannot get his feelings out quick enough which results in a fight. He has to resort to repetition, calling Stradlater the same name repeatedly. This demonstrates Holden's arrogance/insecurity.