Each team has six players on the field. The starting shape, from left to right, is left wing, left link, left middle, right middle, right link, right wing. Most teams play with attacking and defensive specialists at the same positions, but everyone has to be able to fill any role.
Middles control the tempo. They are the central decision-makers when the team has the ball, the ones who most often play dummy half off the rollball, and the loudest voices on defense. A good middle reads the speed of the game and decides whether to go fast or slow it down.
Links sit between the middles and the wings on each side. They are often the targets of attacking plays because they can either run themselves or pass either direction. On defense, links are critical decision-makers because they have to choose whether to come up hard or hold for an outside attack.
Wings play the sideline. On attack they finish the play — they get the ball with space and beat the last defender, usually in the corner. On defense they are responsible for the touchline, preventing overlaps, and chasing back when broken.
The dummy half is a temporary role, not a fixed position. Any player who picks up the ball off a rollball becomes the dummy half — and stays the dummy half only until they pass it on. The role moves with the play: the same person won't be dummy half on every touch, and most players will fill the role several times across a single set. Most often a middle takes it because they're nearest the rollball, but anyone can.