UmbracoIdentity.OAuth is an add-on library to UmbracoIdentity providing an OAuth2 endpoint from which you can authenticate Umbraco Members. The reason to do this is to allow remote / authenticated access to your custom API controllers, such as in the scenario of developing a web/mobile app.
PM> Install-Package UmbracoIdentity.OAuth
There's also a nightly Nuget feed: https://www.myget.org/F/umbracoidentity-oauth/api/v2
There is also just an UmbracoIdentity.OAuth.Core package too in case you just want the binaries
PM> Install-Package UmbracoIdentity.OAuth.Core
Please read and follow the configuration instructions displayed on post package install. At this point in time, configuration is a manual process, so please read the instructions carefully.