A proper fit is setting the correct saddle height, saddle to bar distance, saddle to bar drop, reach, etc. for optimal efficiency and comfort. You can ballpark it by eye but good fitters will do a lot of measurements and try out different setups to see what works best.

The 'basic' shop fit is just raising the seatpost to something that looks about right and maybe swapping out a stem if it's obviously wrong.

A good fit can make the difference between hours of comfortable riding and agony.