A small, shallow sore inside the mouth or at the base of the gums. A canker sore is a small, shallow open wound (or ulcer) in your mouth that can make eating and talking uncomfortable. Canker sores, also called aphthous ulcers, are small, shallow lesions that develop on the soft tissues in your mouth or at the base of your gums. Canker sores are small painful sores on the inside of the mouth. A canker sore isn't contagious. Canker sores are painful sores inside the mouth.