..."tokens" [are] a virtual currency that you can redeem for eBook downloads. It will say 2/2, or some other configuration, which means you have 2 tokens for this week, and you have 2 tokens remaining.
Different books cost different token amounts. The token amount required for a book is labeled clearly on the book display. Unused tokens roll over to the following week. Every 4 weeks, unused tokens expire.That's from the "Freading" page of the Salt Lake City Public Library, describing the restrictions on access to eBooks that are available to library patrons.
Used to be that books had a built-in limitation - as a physical item they could only be read by one person at a time, and copying was arduous and expensive. Providing content for free, public libraries have traditionally posed a threat to the profits of booksellers. For decades the threat was muted by the difficulty of passing the physical object from person to person with any sort of dispatch - getting to keep the object for almost a month meant only 12-20 people could consume any single book in a year.
The digital age has made information easily and readily available, and libraries could now provide unlimited virtual versions of books to patrons at little or no cost to anyone. Those restrictions have continued, only now they are artificial. Libraries, once enthusiastic purveyors of knowledge, are now cast in the odd role of guardians and gatekeepers, making sure you don't consume more knowledge in a week than you deserve to.