brb is an acronym for Be Right Back.
People use it in online games and chatrooms mainly, teenagers use it when they really gotta jerk off, while younger kids use it when they must eat or they parents yell at them.
randomguy: uhm, brb!
randomgirl: leaving me? :<
randomguy: juts gon.nannfnps
*Thypical guys when they write with one hand and having some fun with the other*
randomkid: BRB, XD
randomperson: okay, we'll wait.
*4 hours later*
randomperson: Come on guys, he'll probably be back soon, just wait plz