The fork
One piece attacks two at once. Your opponent only saves one.
A fork is when a single piece attacks two (or more) things at once. Your opponent saves one; the other falls. The knight is the king of forks, because it attacks in an L — squares no other piece covers — and that is why it is the hardest to see. Look at the marks: from c7 a knight would attack the king on e8 AND the rook on a8 in the same move. And when a fork catches the king there is no argument: check is compulsory, and the other piece is left behind.
Put the knight on the square that attacks both black pieces at once.
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