47 pre-thinned colors formatted for the airbrush rather than the brush.
What the Air range adds
Air is Citadel's airbrush range, 47 colors thinned and formulated to spray without clogging. Many of the names echo the Base and Layer ranges, Abaddon Black, Macragge Blue, Mephiston Red, because the idea is to give you familiar Citadel colors ready to load straight into an airbrush cup rather than thinning pot paint by hand.
The spread covers the essentials rather than the whole Citadel palette: a solid run of blues, reds, greens, and yellows, plus the blacks, whites, and greys you need for priming and zenithal work, which is where an airbrush earns its keep. A handful of clear colors are included for glazing and tinting over metallics.
Whether you need this range depends entirely on your tools. If you brush-paint, you can skip it; the same colors exist in Base and Layer. If you airbrush, pre-thinned paint saves a fiddly step and gives more consistent results than eyeballing your own ratio. Note that this range is meant for spraying, not brushing at pot consistency.