In Internet culture, shitposting or trashposting is the act of using an online forum or social media page to post content that is satirical and of "aggressively, ironically, and trollishly poor quality". Shitposts are generally intentionally designed to derail discussions or cause the biggest reaction with the least effort. It may even sometimes be orchestrated as part of a co-ordinated flame war to render a website unusable by its regular visitors.
When it comes to "shitpost music" as a genre, it's a loosely defined collection of tracks and releases created with some of the above mentioned goals in mind. Shitpost music usually contains whichever memes are popular at the time. Production tends to be of poor quality, with overwhelming loudness and chaotic structures.