Mucilage
also: honey layer
The sticky, sweet fruit layer clinging to the bean; how much is left on largely defines the process.
Mucilage is the sticky, sugary layer of fruit pulp that clings to the coffee seed once the outer skin is removed from the cherry. It feels tacky, almost gluey, and is rich in sugars and pectin. What a producer does with it is one of the central decisions in processing.
Why it matters: how much mucilage stays on the bean during drying is essentially what separates the main methods. Washed coffee ferments and rinses nearly all of it away for a clean cup. Honey-processed coffee leaves some on, trading clarity for sweetness and body. Natural coffee dries with the whole fruit, mucilage and all, soaking the bean in fruit flavor.
In the cup, the more mucilage left to dry against the bean, the sweeter, heavier, and fruitier (and potentially funkier) the coffee tends to be. So when a bag lists its process, it is really telling you how its mucilage was handled. The sugars here are also what feed the microbes in fermentation and anaerobic methods.