It's an acronym telling you which cards you should pick. Cards fall in one of five categories, and you should (usually) pick the card from topmost category (B > R > E > A > D) which is still left in the pack.
In a nutshell:
- Bombs, powerful cards that win a game if unanswered. These can be something that generate an advantage every turn (e.g. The Immortal Sun), planeswalkers, or simply overly-large creatures for the mana cost (e.g. Lyra Dawnbringer).
- Removal, cards which destroy/exile/damage creatures (said bombs, but of course they can be used earlier in the game as well).
- Evasion, creatures with special abilities (e.g. flying) which make them harder to block.
- Aggro, smaller creatures which give you early presence on the board and make sure you can play a card almost every turn.
- Duds, cards you don't want in your main deck but might be useful in the sideboard.
magic draft bread gives some articles which explain everything in detail, including examples for each category in the current expansions.