The rook takes along its row and its column, as far as the first piece it meets. In this drill you tap every black piece the white rook 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. On medium and hard, some pieces sit behind others where the rook cannot reach.
Tip: look along the four lines from the rook, one at a time. The first piece on each line is the one it can take.