The rustics play a significant role in The Return of the Native. Elucidate. How do the rustics contribute to the action of the plot?

Your question could be interpreted in a couple of ways; so I've listed below examples of those interpretations below.

Rustics (in reference to rustic elements): In Native, Hardy devotes much of the first section of the novel to describing the rustic heath upon which life for Eustacia, Wildeve, and so many other characters centers. The heath, especially the lighting of the fires, represents not only the countryside way of life and traditions, but it also symbolizes Eustacia's isolation from the urbane life she so desires.  In that sense, the rustics are all-important to the novel's plot because Eustacia's longing to alienate herself from the traditions and manners of the heath is the novel's main conflict.

Rustics (in reference to specific characters)--While Hardy does include several rustic characters in the novel, none is more important to the plot than Diggory Venn, the reddleman.  Diggory is a commoner who longs for Thomasin's love.  Because of his love-from-afar, he is involved in and advance most of the novel's subplots and affects its main plot. He tries to foil Eustacia's relationship with Wildeve (Thomasin's husband); he observes much of the novel's actions and uses his observations to influence the actions of other characters, and in the end, he "saves" Thomasin.