That means that if the Internet Archive and its partner libraries have only one copy of a book, then only one patron can borrow it at a time, just like other library lending.
Lending and renting stuff is not piracy! Many corporate suits want people to start believing this. but i remember going to the library and renting books, movies and games. it was not piracy back then, and it wont be now.
Please go to archive.org > Books > Books to Borrow
Select any book which strikes your fancy. You will see a reading excerpt, like flicking through pages in a library. if you have a free account, you can lend it for 1h at a time.
Or look at this video https://dn720701.ca.archive.org/0/items/openlibrary-tour-2020/openlibrary.mp4