Honey bee robbing is the act of foragers from one or more colonies looting honey from another, usually weaker colony often occurring during a nectar dearth, times of limited and reduced forage availability in the area. An extra high amount of activity around the hive's entrance(s), with fighting and dead bees all around is a sure sign of robbing.
Robbing often leads to a dead out, due to starvation and/or the queen bee being killed, leaving the hive to dwindle to a slow and ugly demise. Anti-Robbing Screens are available, but are no longer thought to be as effective as we would hope. There have been multiple field trials that show they can actually hinder as much as help.
Keeping entrances in the smallest opening position is recommended, but does not always prevent the hungry bees from overtaking guard bees. Keeping strong, healthy colonies with high populations and small entrances is the best defense.
(see video below)