Please suggest the word which means "understandable by specific group". I just forgot that.
You may be looking for the word shibboleth. From Wikipedia:
A shibboleth ( /ˈʃɪbəlɛθ/ or /ˈʃɪbələθ/) is any distinguishing practice that is indicative of one's social or regional origin.
It is also used to mean words used by a members of a group to distinguish themselves as members of that group. When used this way, it is being used metaphorically.