The Redbelly Mine has been overrun by Frostbite Spiders. Fortunately, no one has been hurt, but the spiders are keeping the miners from their work and so the flow in the settlement. Filnjar asks the Dragonborn to clear them out.
When given the quest the Dragonborn must make it to the Redbelly Mine in the settlement. Inside there will be five spiders which must be killed before any progress to the next stage. After that he'll need to return to Filnjar and claim his reward.
The spiders will respawn over time. This result in a exploit as every time the mine is cleared of the respawned spiders the quest's last stage, "Return to Filnjar", will activate and, if visited, Filnjar will pay the reward again.