Folk science is pre-scientific ideas about how the world works. For example, a common idea in folk physics is that an object in motion requires a constant force to stay in motion, and if the force stops being applied, the object will soon come to a halt.
Now, folk science is not always wrong. It actually tends to be very good at predicting what happens in everyday circumstances. If you're pushing a cart, the carts stops moving when you stop pushing. It's when you leave everyday circumstances that folk science fails.
I think the same idea applies to morality. Folk morality is what people generally use when making moral decisions. It doesn't have any kind of rigor or theory behind it, but in everyday circumstances, it works alright. Don't lie, don't steal, don't kill.
The biggest problem with this idea is that folk science is based on things that can be directly observed. Folk morality doesn't seem to be. As a result, it's much more prone to differ between cultures and eras. For example, two hundred years ago, slavery was common and accepted, but it isn't today.