Freedom is a theme in the book because the people are enslaved by the Tripods, but there are some who are free.
In this society, all humans are slaves to giant machines called Tripods. They are controlled by something called capping. No one knows where the Tripods came from.
There are two stories about them. One is that they were machines, made by men, which revolted against men and enslaved them….The other story is that they did not come from this world, but from another. (ch 2, p. 33-34)
The Tripods do not have dominion over the entire earth. In the mountains, the men escaped because the Tripods stick to low ground. Life in the mountains is not difficult, but it is not slavery.
And another thing is certain too: in the end, we will destroy the Tripods, and free men will enjoy the goodness of the earth… (ch 10, p. 214)
At first it is surprising to Will. He never thought to question the existence or domination of the Tripods. When he sees how things once were, it is a shock he does not quite know how to handle. In the end, he realizes that humans can be free, and don't have to accept slavery, but it takes a huge amount of will and character.