Free-air mounting of subwoofers is where subs are mounted to a board attached to the rear deck or placed in the boot against the rear seat.
The boot of the car acts as the box enclosure which houses the subwoofer and isolates sound from the back of the speaker. This solves the problem of sound cancellation due to not using a box.
Free-air mounting saves space and has a flat frequency response. The subwoofer must be specifically designed for free-air use. This method of installation makes them more convenient to install, but their power handling levels are usually much lower than their boxed alternative.