It means the AWS warning you will have just recieved does not apply to you. I only know of them on branches. As the magnets are always live when a signal is on, it can't be turned off, when you pass it in the wrong direction, such as on a branch, you still get an aws warning. It's just so drivers don't get too confused
In most instances the AWS magnet would be suppressed for trains travelling in the opposite direction so that no indication sounds inside the cab. However there are locations where this is not the case, so an AWS Cancelling Board is put up 200 yards beyond the magnet so that the driver knows that the indication received does not apply to that train.
O L Leigh