An Olla is a pot- in this case it's unglazed terra cotta- which is buried in the ground and then filled with water. Plants are planted around it and the water seeps out of the pot as the soil dries out. It's an irrigation system!
Here is a YouTube Video from Laurie explaining how it works.
I found lots of other videos on YouTube about this type of irrigation. A lid is important so the water doesn't evaporate and mosquitoes don't lay their eggs in the water. There are also videos showing ways to make automatic filling systems into the Ollas.
This is all very good for economic water usage. Apparently it's a system that has been used for centuries in North Africa, where water is very precious.