With respect to a rope or a chain or anything mechanical, it means a sudden pull, or a motion that starts and stops.
"Ahab first paid out more line: and then was rapidly hauling and jerking in upon it again."
"[Santa Claus] filled all the stockings, then turned with a jerk..."
"Use this software to smooth out your jerky video footage..."
With respect to a person, it's an informal, insulting word. It means someone who is socially clumsy, annoying, or contemptible.
"People who try to use italki as a dating site are jerks."