An aire is a designated stopover for motorhomes. The full French name, aire de services pour camping-cars, is more useful than the short one, because it tells you what the place is for: services, and a night’s parking.
The word changes as you cross borders. In Germany it is a Stellplatz, in Italy an area di sosta or just a sosta, in Spain an area de autocaravanas. The idea travels better than the vocabulary does.
What you actually find
Most aires are a levelled bit of ground with marked bays, a drinking water tap, and a drain in the ground for emptying waste tanks. Some add electrical hook-ups. Many add nothing at all. There is usually no reception, no barrier and nobody to ask, because the model assumes you know what you are doing.
Payment, where there is any, is by machine: a card, a coin, or a token bought from the tourist office or a shop. Plenty of aires are free to park on and charge only for water and electricity. Some are free outright.
What an aire is not
It is not a campsite, and the difference is the thing to internalise. A campsite sells you a pitch and expects you to settle in. An aire sells you a night and expects you to leave in the morning. Awnings, chairs, tables, windbreaks and washing lines belong at the first and not the second.
It is also not a place for a tent. Aires are for vehicles you sleep in. If you are in a tent, what you want is a campsite or, in some countries, a legal bivouac.
Why they exist
Because a passing motorhome spends money in the bakery, the bar and the supermarket even when it pays nothing to sleep. French communes worked this out first and built aires as tourist infrastructure. Once you see them that way, the etiquette makes sense: you are a guest of the town, for one night.