Not content with just one movie-within-a-movie—you know, The Dancing Cavalier—Singin' in the Rain ends with the suggestion of another movie-within-the-movie: Singin' in the Rain.
Wait—what? If you're a little confused, that's to be expected. It's all very meta.
Let's recap: At the very end of the movie, after Lina gets exposed as a fraud and Don and Kathy reunite and serenade each other in front of everybody at the Dancing Cavalier premiere (that audience really got its money's worth, didn't it?) we cut to Don and Kathy standing in a field and admiring a billboard advertising a new Monumental Pictures film. It's called Singin' in the Rain, and it stars Don and Kathy.
At first, it might seem weird and unnecessarily complicated for the film to be so self-referential, but if you think about it, it makes sense. Singin' in the Rain is a 1952 movie-musical all about the movies, including musicals. As a satire, it deconstructs the movie trade and finds humor in the truth about the industry's shortcomings. In other words, Singin' in the Rain is all up in the movie business's, well, business—so why not end the picture on a reference to the picture itself? It punctuates the satirical nature of the movie.
Not buying that? Look, if nothing else, it reminds you what the name of the movie is, so you can go tell all your friends about what you just saw. Or ease the pains of your Memento-style anterograde.