Laezh-ribod, as it is known in colloquial Breton, or Lait Ribot, which is a more contemporary form of the name, is actually buttermilk made according to the traditions of the old Britany. The recipe for laezh-ribod is very old, being prepared in Brittany for thousands of years already. It is believed that this type of fermented milk appeared back in the days of the Gauls. Probably it started to be made and consumed because no one wanted to throw away the liquid that was left behind after cream was beaten into butter. So, yes, laezh-ribod is actually low-fat butter in a liquid form, being almost as nutritious as butter and having the same charming taste. The fact that is rather available and cheap to make, this type of dairy product started to be consumed a lot around Brittany.
So, yes, laezh-ribod or lait ribot is the fermented liquid resulted from the churning of butter. It has a slightly sour taste and a sparkly characteristic, as small bubbled can be seen at its surface, due to the light fermentation process. Some compare it with drinkable kefir and if you visit Brittany, you will be able to order it in creperies, as it is the kind of dairy product that works well with a portion of soft and fluffy crepes. But it can also be consumed as it is, although it may be rather difficult to find packed in bottles and sold in stores, just like kefir, or it can accompany various dishes, like a potato meal.