Double check
A double check is a check given by two pieces at once, usually a discovered check plus a check from the piece that moved.
You cannot block two lines with one piece, and you cannot capture two checkers unless they happen to sit on the same square. The king has to move.
Nf7+ checks with the knight, and the rook on e2 is uncovered at the same time. Black cannot take the knight or interpose on the e-file; those replies stop only one of the checks.
Double checks are common near the king in open positions. If a discovery is available, look for a landing square that also attacks the king.