In "Oranges," what does the woman behind the counter understand?

"Oranges" is a touching narrative poem by Gary Soto in which the speaker tells the story of when he was twelve years old and went on a sort of date with a girl where he bought her candy from a drugstore and they walked holding hands.
At the very beginning of the poem, the speaker tells us that he is "weighted down with two oranges in my jacket." When he and the unnamed girl reach a drugstore, he tells her to pick out what she candy she wants, implying that he'll buy it for her. Unfortunately, she picks out a piece of candy that costs a dime, and he only has a nickel.
When he goes up to the counter to pay, without saying anything, he puts his nickel and one of his oranges on the counter, silently hoping that the cashier will accept the orange as payment. He then tells us:

When I looked up,The lady's eyes met mine,And held them, knowingVery well what it was allAbout.

The woman behind the counter has clearly seen that the speaker is buying candy for the girl he's with. She understands that this is an important moment for him and that he would be embarrassed if he had to admit that he didn't have enough money. In the following stanza, the speaker mentions the girl unwrapping her chocolate, so we know that the woman behind the counter accepted the orange as payment for the candy out of kindness toward the speaker.