Here's what an online herbal dictionary says:
Marshmallow leaf is used internally to treat inflammation and ulceration of the digestive tract, oral and pharyngeal mucosa with associated dry cough. It relieves irritation of the mucus membranes of the mouth, throat and gastrointestinal tract and helps with respiratory complaints including bronchitis and asthma.
The polysaccharides form a protective film over inflamed and irritated mucosal tissue.
I take it in capsule form and purchased it at the health food store.