Any creation that is not protected by copyright law belongs to the public domain. This gives everyone (the "public") the right to use and edit the work in any way they choose without seeking permission first. Once something is in the public domain, you are free to use it anyway you'd like. Websites like Project Gutenberg, Internet Archive, and HathiTrust have large digital collections of public domain works you can use and remix.
Creations generally enter the public domain because the copyright has expired or the creator dedicated the work to the public domain. Under current US law, the length of time a work is protected by copyright depends on several factors. If the work is unpublished or published after 1978, the copyright will generally last until 70 years after the creator's death. For works published before 1978, the copyright generally expires 95 years after its first publication. For a more in-depth look at how long copyright terms last, visit the link below.
If you think 95 years after publication is a long time for copyright to last, you're right! Copyright terms haven't always been as long as they are today, and a lot of that has to do with corporate interests. Adam Ruins Everything gives a brief overview on how and why copyright lengths have increased in the video below.