It means the airline has selected the owner of the boarding pass for secondary security screening. There are a number of factors that will cause someone to be flagged for secondary screening, but of course they won't tell us exactly what they are. Speculation shows the following are influencing factors:
- How the ticket was purchased (cash, credit card etc)
- Where the ticket was purchased
- How long before travel it was purchased
- Country of residency/citizenship
- One-way vs round-trip ticket
- Membership of the airline's FF program
- Direction of prevailing wind at the time of ticket purchase
It is all pre-determined by the airline's computer system before you check in and is not manually triggered by the check-in agent. I believe it cannot be changed by the check-in agent. There is a very good chance that if they computer has selected someone once, they will be selected again if the tickets are purchased in the same manner.
I used to get it regularly when travelling on ATW tickets (seen as a series of one-way flights), but did not get it at all on my last trip when using my AA FF account instead of my QF account for the flights.