In text messaging or online chats, "mb" commonly stands for:

• "My bad" - an informal way to admit a mistake or apologize.

For example:
Person A: "You forgot to send the report."
Person B: "mb, I'll send it now." Depending on context, "mb" can sometimes mean other things (like "megabyte" in tech contexts), but in casual texting, "my bad" is the most frequent meaning.