Dishwasher Spray Arms are the units that direct and distribute the wash and rinse water to all parts of the dishwasher compartment. Most machines have two spray arms and some have more. The top and bottom spray arms may look the same but it is important not to get them mixed up.
Over time, Spray Arms can get blocked by food deposits as well as limescale deposits particularly in hard water areas. This will lead to not getting the cleaning performance that you require.