If it were White's move he could give check with the Rook on e8.
But Black could take the Rook with the Knight. He would naturally
do this instead of either moving out with the King to h7 or
interposing the Bishop on f8.

If a King is in Check and there is no move with which to get him
out of it he is said to be "checkmate" and the game is ended.
Diagram 6 shows an example in which either player can give
checkmate on the move.

If it were White's move he would take the Pawn on g6 with his
Queen. Now Black's King is in check as White's Queen threatens to
take him on the next move. The King cannot move to either g7 or
h7, for these two squares are also commanded by White's Queen.

+---------------------------------------+
8 | | | | ^R | | #Kt| #K | #R |
|---------------------------------------|
7 | #P | #B | #P | | | #P | | |
|---------------------------------------|
6 | | #P | #Q | #B | | | #P | |
|---------------------------------------|
5 | | | | | | | | |
|---------------------------------------|
4 | | | | | | ^Kt| ^Kt| |
|---------------------------------------|
3 | | ^B | | ^Q | | | | ^P |
|---------------------------------------|
2 | ^P | ^P | ^P | | | | ^P | ^K |
|---------------------------------------|