Interference
Interference is putting a piece on the line between a defender and what it defends, so the defense cannot get through.
Rooks need a clear file, bishops a clear diagonal. A single unit dropped on that line cuts the connection. The interfering piece does not have to capture anything.
The rook on a8 defends the bishop on a2. Na5 steps onto the a-file. The rook no longer sees a2, and Rxa2 wins the bishop.
Interference also appears as a defensive idea: you can step onto a checking line with a piece that also attacks something, so the opponent cannot take for free.