To alert others of the presence of Law-Enforcement (police or correctional officers).
Originating in United States prison systems most likely from the Spanish word "jurar" "to swear" as in court or "jurado" referring to the jury.
While sometimes someone may call out "jura!" Many times a Jura Call may be non verbal, multiple flushes of a toilet, a loud cough or even a whistle.
The flushing of toilets is a fairly common Jura Call since inmates may need to "flush" their contraband.