The whole poem is one long sentence that builds toward a single question. It starts with an ordinary, slightly annoying detail: the heat is so bad that the air conditioner in an old car does nothing for the first few minutes, so you have to roll the windows down. Then, in the last line, the speaker asks, "would you like to have sex in the swimming pool?"
The shift is sudden, and that's where the charm lies. The speaker sounds relaxed, a little playful, and not at all nervous about asking. The question comes out of the heat as if it were the natural next thought. When it's too hot to think, why not cool off in the most fun way possible?
The mood is flirtatious, laid-back, and a bit funny. The everyday, sweaty details make the invitation feel casual and real rather than glamorous. The title, "The Season," suggests summer itself, with its heat and restlessness, is nudging the speaker along.