Rune-Sign Language was a non-verbal manner of communication that Dark Wizards began to use during the Second Wizarding War in the 1990s. It involved making specific hand gestures to represent a certain sequence of Runes.
The Ministry of Magic managed to crack the code. It disseminated a Wizarding Public Awareness Notice alerting the wizarding community that ill-intentioned wizards were using the signs to "bamboozle bonafide witches & warlocks!" and published Manual M39h-z to assist them in breaking the code themselves.