The description offered via IMDB is a two-sentence collapse of the story: "An orphan goes to live with his free-spirited aunt. Conflict ensues when the executor of [the orphan's] father's estate objects to the aunt's lifestyle."
In these two scenes, excerpted from the end of the movie, the orphan, Patrick Dennis, has grown up and has chosen his fiance from one of the executor, a conservative banker from the Knickerbocker Bank ("So conservative, they don't even pay interest!") wealthy clients' family. In the first scene, Patrick's free-spirited Auntie Mame is meeting her prospective in-laws for the first time, driving up in her Dusenberg from her Manhattan Beekman Place apartment, to the Upsums 1950s suburban home in an exclusive Connecticut community.
In the second scene, Mame returns the Upsums hospitality with "an intimate family dinner," at her Beekman Place pad that pulls out all the stops for tips & techniques on how to run-off potentially aggravating in-laws, before the marriage. It is also a great scene because Mame torches the bigotry of the Upsums and "conservatives," exposing them for the "shallow, vain, addlepated bigots" that they are.
When Mame shouts "Jackpot!" towards the closing, it's evident that she's gotten exactly what she was after all along. But, as women have had to do for eons, Mame's path to victory is not direct or obvious, but instead utilizes all feminine means of persuasion, of the very type that has confounded men, especially conservative men, for just as long.