Ranula is a fluid collection or cyst that forms in the mouth under the tongue. It is filled with saliva (spit) that has leaked out of a damaged salivary gland.
Salivary glands are small structures around the mouth which make saliva. Saliva should drain from these glands directly into the mouth. If one of these glands is damaged then the saliva leaks out into the tissues next to the gland forming a cyst or bubble near the gland. This cyst is called ranula.
If the ranula stays in the mouth underneath the tongue it is called a simple ranula and if it grows down into the neck under the mouth it is called a plunging ranula.