Lacto-fermentation happens when lactobacillus bacteria convert sugars (lactose, others) into lactic acid. Lactic acid is a natural preservative that inhibits the growth of harmful bacteria, preserves or increases the vitamin and enzyme levels, and improves digestibility.
The good lacto bacteria are all around us. To encourage fermentation, we create excellent conditions for this natural process to kick off and go into high gear. If you want to raise chickens, or keep tropical fish, you create the right living conditions. Exactly the same with the lacto-bacteria. Submerging vegetables in salty water is all it takes.
What happens is, the water keeps out the oxygen that the lacto doesn’t like, and the salt kills off bad bacteria. This allows the lacto guys to start converting sugar in the veggies into lactic acid, and the lactic acid acts as a preservative.
(By the way, this is exactly the same basic principle at work when you try the sourdough bread course.)