This is a water-seal stoneware crock. The design is ancient.
It is, essentially, a large ceramic vessel that you put vegetables and sometimes brine into. To prevent spoilage, you place those ceramic weights on top of whatever food is in the crock, and that keeps them weighted down, below the level of the water. Because fermentation creates gases, most crocks have a "water groove" in them. The lid sits in the groove, which allows air to escape but not come in. Because fermentation creates gas, the interior of the crock is positive-pressure, and because the gas created is almost entirely carbon dioxide, it's a low-oxygen environment that additionally helps prevent spoilage.
And all this would be pointless without lactobacillus, the bacteria that chomp down on the vegetables you put into the crock. They're anaerobic, which means totally fine without oxygen, and they produce an environment that's inhospitable to most other organisms. The main things they produce are CO2, which means no oxygen for other bacteria, and lactic acid, which makes the fermented thing sour and also decreases the pH low enough that many other bacteria cannot survive. They tolerate high levels of salt, which kill yet more competitor bacteria. It ends up being a really really good way to keep food from going off.
Our ancestors figured this out thousands of years ago without knowing what bacteria were. This general ceramic design has been in use around the world in virtually every place that had ceramics, salt, and too much cabbage or cucumbers that was going to rot if they didn't do something about it. It's thousands of years old, so old that it gets hard to interpret the evidence of the ceramics.
And I have crocks like this in my kitchen, where I make my own ferments, and I always think about how beautiful and elegant it all is, and how this was probably invented hundreds of times as people converged on something that Just Works.
(I do have pH testing strips though.)
