Definition of scary board
A "scary board" is a board that is scary. This is usually understood to mean that the board has several draws and possibilities. If the board is A-J-T-9 with four spades, it is a scary board. On a scary board, players will usually be cautious. Here, it is possible that one or more players have a straight, two pair, a flush, a three of a kind, etc. A scary board is even more frightening when there are several players in the hand.
A scary card can turn a board into a scary board
Let's take a flop like 7-T-J with two spades. Here, the board offers several draws. Now let's imagine a turn that would be a "scray card" and turn this dynamic flop into a scary board: an 8 of spades. With the 8 of spades, our opponent could have several 2 pair combinations, he could have several straights, he could have a flush. A scary card is a card that makes the board scary, i.e. a board where the possibilities are enormous. On the one hand, when a player bets on a scary board or a scary card, you have to give him credit. But on the other hand, a scary board or scary card is an excellent candidate for a bluff. If you bluff a scary card and your opponent has nothing, he will often give you credit and fold.