Mucositis, the inflammation of the mucosal cells in your mouth become red, inflammed, and very painful. It can be caused by several things, such a:

Not caring for dentures properly (wearing ill-fitting ones or not taking them out often enough
Medications that cause dry mouth and/or mucositis
Xerostomia (dry mouth)
Infections
Malnutrition
Certain illnesses
Side effect of chemotherapy or radiotherapy

We’ve discussed dry mouth here before (Many Medications Cause Dry Mouth , Painful dry mouth from medications ) but mucositis is more than just dry mouth.
According to the article, Managing Oral Mucositis in Patients With Cancer,