This story is about three girls who walk into an A&P grocery store on a Thursday afternoon, wearing only their bathing suits. The reaction they get from the other customers makes up the bulk of the tale. Ultimately the manager of the store confronts them directly, going so far as to say that they are not dressed "decently." Sammy – the story's narrator, and a clerk in the A&P – is a consummate people watcher, skilled (or so he thinks) at judging people based on how they dress and act and on what they buy. This very short story, written in 1961, gets us thinking about all the rules and norms regarding fashion, and the greater freedom we have to express ourselves through fashion today.
Lengel is right: the girls shouldn't be allowed to wear bathing suits in the grocery store, because they might offend other customers.
The fact that Sammy thinks most people are sheep shows that he puts too much value on appearances and doesn't take the time to get to know people.