It's basically the digital library catalog, accessible online to anyone, that lets you:
* Search for books, journals, and other resources held by the library.
* Browse the catalog by subject, author, title, keyword, etc.
* Check the availability of items and their location in the library.
* Place holds on items that are currently checked out.
* Manage your library account (check out history, renew items, etc.).
Think of it as the digital equivalent of the card catalog that used to be in physical libraries.