Ancestry Library is a genealogy service your library offers within the library. You must be in the library to access the service. You will not be able to access it from home.
Ancestry Library is a collection of more than 9,000 databases and 200 billion images from the United States, United Kingdom, Canada, Europe and Australia. You can search through these databases in the library and send a link to yourself to use outside of the library to access the information that you have found.
This tutorial focuses on using the website itself. For questions about genealogy, Ancestry has a YouTube Channel with many videos that will help you find the genealogy information you are seeking.
To avoid confusion, it's worth noting that Ancestry Library Edition has some key differences from the individual subscription version of Ancestry. The subscription version, for example, adds the ability for you to access the service from home, create your own family tree, and search more databases.