The explanation in a sibling comment looks great: https://www.tinteddy.com/introduction-to-atcs-what-are-artist-trading-cards

Another one I saw before, kinda official FAQ of the relevant subreddit, is at: https://jenivievedesign.substack.com/p/artist-trading-cards-etiquette-101

Another cool one I saw is the zine about ATCs: https://www.betty-draws.de/2026/07/15/atc-zine-07-2026

In shortest words, it's hand-made cards you do in 2.5x3.5in size (6.4x8.9cm) in any medium you like. (It's a standard "trading cards" size, like Pokemon or MTG or baseball cards in the USA, so you can easily store them in plastic sleeved catalogs.) You can then swap them with other artists, in person or over the internet, or just keep them for yourself. They are not to be sold (if you want to sell, you mark them as ACEO rather than ATC, it's the same size but a different, adjacent community and idea). There are online (and I think also offline) communities for swapping or just showing your collection.