The knight jumps 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 piece the white knight could capture 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 pieces right next to the knight, where it cannot reach.
Tip: a knight always lands on the other colour from the square it stands on. If a piece is on the same colour as the knight, it is safe.