A croquet player on Court 6 at Cheltenham Croquet Club.
To play a croquet stroke, the player needs to hit one of the other balls with the ball they are playing. The ball that they played is then picked up and placed in contact with the ball that was hit. The ball in play is then struck in such a way that both balls move - this is the 'croquet' stroke.
There are various types of croquet stroke - this lady is playing a 'roll' stroke, where she has struck the yellow ball so that both it and the black ball with which it was in contact are both moving: the outcome of a roll stroke is that both balls should move a more or less equal distance.