In a nutshell: a lot. You need to ensure all screws are tight from time to time. Belts need to have enough but not too much tension. Bearings need cleaning and

In a nutshell: a lot. You need to ensure all screws are tight from time to time. Belts need to have enough but not too much tension. Bearings need cleaning and re-lubing. Leadscrews ditto. Nozzle needs unclogging and replacing every now and then. Fans must be kept as dust-free as possible. Print bed needs wiping frequently with 99%+ isopropanol, eventually a wipe with acetone (if PEI; glass only needs alcohol but you need to ensure it's squeaky clean). Plastic parts can break or sag and need reprinting and replacing. I'm sure I've missed something but you get the idea.