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.