A work is in the public domain when copyright has expired or never applied. Anyone may read it, copy it, share it or build on it, without permission and without payment.
Copyright lasts a long time but not forever. In the United States, works published more than 95 years ago are generally in the public domain, which is why each January another year of books becomes free to everyone. Terms differ by country, and are often calculated from the author's death rather than the publication date, so a work can be public domain in one country and still protected in another.
The public domain is why a fourteen-year-old anywhere in the world can read Frankenstein or Pride and Prejudice tonight, for nothing, without asking anyone. Nearly everything we mean by "the classics" now lives there.
Every one of the books here is public domain, sourced from Project Gutenberg. Wordema changes how the words are presented — one at a time, at a point where your eye can rest — and changes nothing about the words themselves. There is no charge, no account and no restriction.