TL;DR instead of a box tuned to keep the driver under control, which restricts output, a baffle is used to separate the front and rear waves.
With larger drivers, the mass of the cone becomes a concern as it's inertia can add it's own distortion to the output sound. To combat this drivers are typically placed in boxes sized so that the air pressure & vacuum they build while they operate is used to assist the drivers suspension to prevent unwanted movement of the cone. The compromise though, is that the smaller the box, the more it restricts low frequency output. The bigger the box, the less accurate the sound is.
Some drivers are designed with sufficiently stiff suspensions and can be placed in what is called an infinite baffle setup. You can't just leave a drive to play in open air as the front pressure wave would cancel out the rear negative pressure wave. But an IB is designed to add nothing else but a separation of the two waves, as if there were in infinitely large wall between the waves. Ideally there's no suspension tuning contributed at all. (see how going from one discipline to another can get confusing really quickly
) This means the low frequency output isn't restricted and things can get loud and still be accurate.
IB setups help to cool the voice coil allowing for more power to be passed through it. But they also make it easier to bottom out the voice coil and damage it. Again, lots of compromise.
In a house IB would consist of a VERY large box build for a driver. But in a car, IB setups are typically accomplished by putting a piece of MDF between the rear seat and the trunk, sealing the two areas from each other as much as possible, and just mounting the driver to that wall. They can end up taking up less trunk room then a traditional setup, and either be more effective, or use less power to get the same effect. Less power is nice because it's hard to generate that much power in 12 volt system.
And then there's port tuning for boosting specific frequency ranges, crossover designs to provide phase shift to combat the phase shift from the box design, isobaric chamber designs, folded horns....