IMO the message depends on the scenario. if they come up behind you in the fast lane it means move over pls. if they are coming straight at you it means check your speed. could be a cop, could be a herd of deer, could be anything. check your speed and prepare for something. if you are trying to merge, or turn out of somewhere, it means go ahead. if you are driving a semi and just passed another vehicle it means "you are clear to get back over". flash your trailer lights to say "thanks". this can work for cars as well. if someone is trying to get into your lane. but the typical response is to speed up so they cant get over and people wonder why no one uses turn signals... because the instant they come on, they speed up so you cant get in.