Brb is an acronym for be right back.
Be right back is used to state that one person is leaving a conversation, situation or place to soon return. Usually within a few minutes or hours but never more than a day UNLESS specified.
Example: Fort: "Me and Tay will brb"
You: "Hey man haven't heard from you since you said brb last night hopefully everything is alright"
Fort: "Yeah everything is good, I'm just a dickhead and forgot to mention something else came up so I couldn't come back"
You: "Oh ok that's fine was just worried, usually that means you would be right back so I was here waiting"
Example 2: "Brb real quick going to Italy for a trip with the family"