A bishop attacks along its diagonals, as far as the first piece it meets. In this drill you tap every black bishop 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 add bishops that are almost, but not quite, on a diagonal with your king.
Tip: a bishop stays on squares of its own colour for the whole game. If a bishop stands on the other colour from your king, it can never attack it.