Ollas are unglazed clay jars with lids. You bury them up to their necks, fill them with water, and the water slowly releases into the surrounding soil. They can water in about an 18 inch radius around themselves, regardless of size. They reduce water loss to evaporation and deliver water to the root zone. This is a two gallon olla. I like them best in small spaces like half whiskey barrels or raised beds.
I buy my ollas from Pretty Pots and Beyond.