A knight attacks in an L shape: two squares one way and one square sideways. Nothing in between can block it. In this drill you tap every black knight that could take the white king right now. Find them all and the next board appears.
Get five boards in a row right for a silver star and ten in a row for gold. A wrong tap shows you the answer and starts your count again. Harder levels put knights right next to your king, where they cannot reach it.
Tip: a knight always attacks the other colour from the square it stands on. If a knight is on the same colour as your king, it is not attacking.