An oral mucocele is a harmless, cyst-like (fluid-containing) swelling of the lip or mouth lining (mucosa) due to mucus from the small salivary glands of the mouth leaking into the soft tissue, usually from injury (trauma) or blockage of the gland. Tartar-control toothpaste might be a cause of some mucoceles.
A similar lesion, the mucus-retention cyst, occurs from blockage and backup of saliva in the gland.