'High sign' is a general term used to refer to any hand signal used as a warning or to signal a change of events. Common high signs are thumbs up, a wave, or the A-okay gesture (forefinger to thumb, with the other fingers raised).
Usage is generally something like "I'll give you the high sign if I see anyone coming."
In 1921 Buster Keaton made a silent film called The High Sign (You can watch it here). It appears that the high sign referred to in that film is what we would call a gang sign, used by gang members to identify themselves to eachother. This sense is no longer in common usage.