Do you know someone who put their tongue between their upper and lower teeth when pronouncing s, z and pronounce them as th as in think and that? That’s a lisp.
A lisp (also called sigmatism) is a common and treatable speech impediment involving mispronunciation of sibilant sounds. Sibilants are speech sound produced with a hissing effect, e.g. s, z, sh.
It is common among children but usually disappear before the age of six. However, some have it till adulthood. The main known cause of a lisp is wrong tongue placement.
The three types of lisp are frontal, dental and palatal.