He is saying that they are like spiders who weave webs to survive without knowing that their webs are too weak to withstand real threats: just like the web won't be strong enough to stop a rock falling through it, their good deeds won't be enough to save them from hell fire.
In this sermon, Jonathan Edwards is urging his congregation (and by extension, all of the nation) to repent of their sins and pray for forgiveness before it is too late and they go to hell.
