Plains enclosures are enclosures for animals that live in plains habitats.
You can find plains enclosures in the shop. If you tap on the "Enclosures" heading in the shop, you'll see a number of tabs along the left-hand side denoting different enclosure types. The second tab (with the three bushels of grass) has plains enclosures and suitable equipment for them.
Enclosure equipment can only be placed in the matching type of enclosure. So if you have a "Trough (Plains)" it can only be placed in a plains enclosure, not in a grassland or a forest enclosure.