BRB stands for "be right back." It's used in online messaging and chatting when you have to leave your device to do something else temporarily. It's typically used when you're going to be gone for a short time to do a quick activity, such as going to the bathroom or answering the door. It implies that your conversation partner should wait for you to come back. It's closely related to the acronym AFK, which stands for "away from keyboard."

Source: How-to Geek