Tools, parts, techniques, and procedures to keep your bike in working order. MAINTENANCE is not the same as REPAIR, a separate tag. Maintenance is what one does on a regular basis. Repair is what one does only when something is broken.
Also see this thread in meta for more information on the differences between the repair and maintenance tags.
If your question is about, for example, keeping stuff on your bike in working order or in its best condition (such as your bike's drivetrain, brakes) it should have this tag. Questions about tires or paintjobs are borderline, depending on focus of the question.
This is one of the most popular kinds of question here, as you can see.