Flagbearer is a creature class. Flagbearers are soldiers or civilians who bear a standard or oriflamme, i.e. either a type of flag or an inflexible but mobile image, which is used as a formal, visual symbol of a military unit on the battlefield.
In Apocalypse there were two White Flagbearer creatures (Standard Bearer and Coalition Honor Guard) and an aura (Coalition Flag) that granted a creature the Flagbearer subtype as well as the inherent ability "As long as you have one in play, any spell or ability your opponent plays that can target a Flagbearer must target a Flagbearer". If a spell has multiple targets, only one of the targets needs to be a Flagbearer.