IOB is short for Influx of Bitches, a term used for when a large group or pack of girls enters a room, usually girls famous for being together or else anticipated by the people already in said room. Can also be used in reference to irritating girls, or to hot girls observed by bros.
Guy 1: Yo man, Cammie and all her friends just showed up!
Guy 2: Haha wow, IOB am I right??