What is Lucretius on the nature of things about?

Lucretius, answered from the texts and cited to the page.

Lucretius answers

It is about what the world is made of and what follows from that for the fear in men's hearts. I take two things to exist: body and the void in which body has its place. Where space lies empty, body is not there, and where body stands, there is no empty void.¹ From this I build everything. The true first-beginnings are solid and free from void, and no force can quench them, however hard it is to believe that anything solid can be found among things, when the thunderbolt passes through walled houses and iron whitens in the flame.²

Because they are solid they are everlasting. Were matter not everlasting, all things would long ago have passed to nothing and been born again from nothing; but nothing is created from nothing, nor is what has been begotten summoned back to nothing, so the first-beginnings must be of immortal body, into which all things are dissolved at their last day, that there may be matter enough for renewing things.³

And nature has set a limit to the breaking of things, for we see each thing put together again, and fixed seasons ordained for all after their kind, in which they reach the flower of their life.⁴ These seeds are not still. Through the void they are borne with a speed that outstrips the light of the sun, since nothing checks them from without and each is a single whole with all its parts.⁵

They fall downward by their own weight, and at times quite undetermined, and at spots undetermined, they push a little from their path.⁶ That is how a world comes to be, and how the will is not dragged forever by cause upon cause. Then comes the lesson that the poem is for. Those who suppose the gods appointed all things for the sake of men fall far from true reason.

Even knowing so little of the first-beginnings, I dare affirm from the workings of heaven itself that the nature of the world was not made by divine grace for us, so great are the flaws with which it stands beset.⁷ A world that goes by its own laws and was made by no one for us leaves no room for the terror religion breeds, and where the terror goes, there is peace.

Sources

For wherever space lies empty, which we call the void, body is not there; moreover, wherever body has its station, there is by no means empty void. Therefore the first bodies are solid and free from void.
On the Nature of Things, pp. 43–44
Now the true first-beginnings of things, no force can quench; for they by their solid body prevail in the end
On the Nature of Things, pp. 43–44
the first-beginnings must needs be of immortal body, into which at their last day all things ca be dissolved, that there may be matter enough for renewing things.
On the Nature of Things, pp. 44–45
a set limit to breaking has, we may be sure, been appointed, since we see each thing put together again, and at the same time fixed seasons ordained for all things after their kind, in the which they may be able to reach the flower of their life.
On the Nature of Things, pp. 45–46
the first-beginnings, which are of solid singleness, when they pass through the empty void, and nothing checks them without... must needs, we may be sure, surpass in speed of motion... the light of the sun
On the Nature of Things, pp. 70–71
when first-bodies are being carried downwards straight through the void by their own weight, at times quite undetermined: and at undetermined spots they push a little from their path
On the Nature of Things, pp. 71–72
the nature of the world is by no means made by divine grace for us; so great are the flaws with which it stands beset.
On the Nature of Things, pp. 71–72