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“Tuning” is making the engine work as close to the best it can. Kind of like adjusting a guitar string slightly to get it to vibrate at the perfect (desired) frequency, a multi-cylinder engine like those in cars will have some “best” timing to the cycle of firing pistons. Old time mechanics (way long ago) would literally “tune” a car by ear, by listening to how the pistons were firing and identifying the one, or few, that weren’t firing when they should, and fixing it, until the car engine sounded just right (if they were good mechanics, of course). They did not have electronic tools to measure the system functioning. They did it by ear, and by eye too (looked for unwanted vibrations).
Tuning is making that “Best” timing come to be, as close as you can. Each piston fires in order, one after the other, then back to the one that started it all, doing it again. Tuning is making the pause between each piston firing be the same (or when it needs to be, which might not always be the same for every step), so they always push on the drive rod (the engine cam) in their own turn at just the best time to keep the rod spinning at the same rate without differences between the input from each piston. However, you also need to link in the positions of each piston, which move up and down, so they all hit top and bottom of their individual movement at exactly the same time that the spark is being made, and also according to the same pause length between each piston. And finally, the piston movement is linked to letting gas in and, on the other side, pushing exhaust out, and that has to be made to work the same in each piston, as best as you can get it.
You want all of these different pistons to do each of those steps at exactly the same place in cycle, each and every time. So, you might have to adjust the piston positions, the spark timing, and the gas+air injection (and amount) to get it all to go how it will be the best you can. Each piston moves the same way but in its own turn, perfectly when it should.
Sort of like a multi-man canoe, say. All the paddlers want to be paddling at the same frequency and strength or the canoe will not go straight and smooth and will not get the most from the work of each paddler. Car engines need to do about the same thing, as a general idea: work all together smoothly.
Modern cars have electronic ignition (computerized firing timing) and fuel injection, so there isn’t a lot that can be done to actually tune the car, except maybe adjust the pistons a bit. Don’t really need to “tune” a modern car, but you do need to change spark plugs or maybe clean the system once in a while, so if you get a tune-up these days, that is mostly what is done: check the plugs, check that the computer is doing the job the way it ought to, and make sure there isn’t a lot of gunk accumulating in, or wear happening to, the pistons. In old cars, there were mechanical systems that could be (had to be) physically adjusted for each of those aspects, and they would wear out or vibrate out of place with use, and need to be put back into the ideal condition, every so often.
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