If you have not read this work, please do so. It is one of the greatest works in the ancient world.
The meaning of book one is to set up the rest of the work. It opens with an invocation and prayer to Venus and seeks to show men the true path of happiness. He says that Epicurus first showed the way. It is to shed fear by realizing that the whole universe is made of atoms. When people realize this, they will no longer fear and so be mired in superstation. For example, Lucretius says that superstition made Agamemnon sacrifice his daughter at Aulis.
Here are some excepts from book one, which gives the tenor of the book:
"Nothing ever springs miraculously from nothing... all are formed from fixed seeds... Any given thing possesses a distinct creative capacity... In every case, growth is a gradual process... Things are created from a definite, appointed substance... All things are composed of imperishable seeds... No visible object ever suffers total destruction, since nature renews one thing from another, and does not sanction the birth of anything, unless she receives the compensation of another's death."
Now, is the book philosophy or science? The best way to answer this question is to say that this is a false distinction. In the ancient world, science and philosophy were related and in many ways the same. We must not have a modern view of the world when reading ancient texts. In light of this, Lucretius's work is both philosophy and science (and its poetry - dactylic hexameters).