These glands secrete mucus to facilitate the movement of particles along the body's various tubes, such as the throat and intestines.
Submucosal means that the actual gland resides in the connecting tissue below the mucosa.
Each sac (acinus) has one end that can open and close (dilate) to allow the mucus out.
The acini empty into little tubes (tubules) that lead to a reservoir (collecting duct) that has a portal through the skin (mucosa) that can open and close allowing the mucus into the main tube.
[1] The submucosal glands are a companion to goblet cells which also produce mucus, and are found lining the same tubes.