What exactly is the mucocele under the tounge?

Cyst: A mucocele is a mucous retention cyst that can occur in any area of the mouth where there are minor salivary glands. It occurs when the duct is severed from the gland or the duct is blocked and saliva pools under the soft tissue. A rather large type of mucocele, called a ranula, can also occur under the tongue. This involves one of the major salivary glands in this area.