Fixing something completely random or unspecified. In most cases, if "shit" is replaced with "stuff", the sentences remain equivalent. If a possessive pronoun, like "your" or "their", is used between "fix" and "shit", this changes the meaning of "shit" to something along the lines of "mess", this often is referring to a personal mess, but also could be a social mess or physical mess.
Frank: "My parents aren't home, do you wanna come over?"
Evelyn: "I'm fixing shit right now, but I'm coming as soon as it's taken care of."
Daniel: *spills beans on the floor*
Claire: "You should probably fix this shit before someone slips and gets hurt."
Bob: "I lost all my life savings and the house because of a bet."
Alice: "You seriously gotta fix your shit man."
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.