It's a very old cartoon, one of the first featuring Mickey Mouse goofing around on a small steam-powered boat on an American river.
He starts by pretending to captain the ship, playing with the steering wheel and horns, before he is chased away by the real captain where it is shown he is supposed to be scrubbing the decks. The ship then pulls into a small dock and he picks up Minnie Mouse, who brings a guitar and music on-board with her. A goat promptly eats the music and guitar and Mickey gets the idea to wind the goat's tail like a victrola to play music, and a sort-of musical number commences where Mickey interacts with a menagerie of animals to make sounds that compliment the song. By today's standards, some of the interactions range from amusing, to distasteful and mean-spirited. In the end he is caught again and is thrown into the boat's kitchen to peel potatoes, and throws a potato at an annoying parrot in the window, which is a last minor victory of sorts before the cartoon ends.
Disney themselves cut a scene where Mickey grabs a sow that is feeding its piglets and slaps them away, before prodding at its teats and manipulating it as if it was an accordion. Apparently this was cut not that long after the cartoon aired.
It was interesting but I didn't enjoy watching it a whole lot with today's eyes, I must say.
Anyway, with all that in mind, the joke here is that the video is only the start and end cards, as Steamboat Willie would be quite short without animal abuse. In reality, some might be preserved, but this is a humorous point that there is a lot of it.
