Libraries is a special folder in Windows, introduced in Windows 7. It allows you to create libraries - special folders which can aggregate files from several different folders and show it under a single, unified view. A library is an indexed location, meaning Windows Search will be completed faster in a library compared to a regular non-indexed folder. In Windows 7, when you opened Explorer using the mouse, it opened the Libraries folder. In Windows 10, Quick Access is the location at which Explorer opens so if you wish to add Libraries to the Quick Access folder, here's how you can do it.