The bishop takes along its diagonals, as far as the first piece it meets. In this drill you tap every black piece the white bishop 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 add pieces that are almost, but not quite, on a diagonal.
Tip: a bishop stays on squares of its own colour for the whole game. If a piece stands on the other colour, the bishop can never take it.