In short, the term board in Poker refers to all cards that are publicly visible to the entire table. This means that the board in poker includes in particular the flop, turn and river cards. Every poker Player can use the board at any time to "create" his own winning combination.
Example: If the board shows the cards 7 (heart), 9 (heart), jack (diamond), 8 (heart) and king (heart), then it is enough if the player himself has another "heart card". In this case, he then has a flush.
In this context there is also the statement "Playing the Board". In this case, the respective player does not use any of his hole cards, but only the cards of the board. In German, the term "Board" can be translated as "Brett".