As you can see in the following diagram, bishops can have enormous power on the diagonal. Here, the White Bishop has placed the Back King in check. As soon as the Black king moves, the bishop will be able to capture and win and the Black Queen! We call this a skewer, an bit like a shish kabob.
Let's now take a look at a different kind of bishop skewer in operation. We reach the position in the next diagram from the Queen's Gambit Declined after 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Nbd7 5.cxd5 exd5
Note that the White Bishop on g5 is "pinning" the knight on f6. If the Nf6 were to move, White would be able to capture the Black queen with Bxd8.
White therefore decides to play 6.Nxd5, an interesting mistake that even some masters have made! It looks like white is just winning a pawn, but Black has a VERY strong reply.
As you can see, the Black knight on f6 was pinned, but it could still move! Is Black crazy? Surely white will take the Black queen.
Sure enough, White must take the queen. Otherwise, Black just won a piece on d5.
Here, a very strong reply. It's check from the black bishop. The white King is in check and cannot move! And, white cannot capture the Black Bb4!
So white has no choice but to play Qd2, walking straight into a different kind of bishop skewer. Set up the board and see that black will emerge a piece ahead after all of the exchanges.