Discovered check
A discovered check is a discovered attack that puts the enemy king in check.
Because the king is in check, the opponent must stop the newly opened line. The piece that moved is often free to take material on that same turn — the opponent cannot capture it unless that also blocks the check.
The rook on d1 faces the king on d8. Ne4 steps off the file and the rook checks. Black has to deal with the rook, not with the knight.
If the moving piece also gives check, the position becomes a double check, which is even harder to meet.