A group of males masturbating each other. Not the same thing as a group of males masturbating in each others' presence, or games such as
Come on a cracker.
Jerking is not a common term in the UK where they would be more likely to use the word
wanking.
Also used as a term for a group of people mutually massaging each others egos, see:
The Academy Awards is just a big circle jerk