OpenAthens Keystone is middleware that allows you to use the widespread and simple OpenID Connect software to participate in SAML federations around the world and easily allow institutional subscription to your content.
OpenID Connect (or OIDC) was introduced in 2014 by the OpenID Foundation to deliver a more API-friendly way of performing many of the same tasks as OpenID 2.0. This API-friendly approach is what makes it work well within native and mobile applications as well as OpenAthens. It is, basically, an authentication layer on top of the OAuth 2.0 authorisation framework.
There are versions available for all major platforms and many minor ones; most are quite straightforward to implement. If you are already equipped for logins such as Google accounts, you may already be using it.
At the basic level, OpenID only does authentication - i.e. is the user who they say they are; OAuth is for finding out information about the user such as identifiers. OpenID Connect enables the authentication and the retrieval of information you can authorise on in one compact package.
OpenAthens (originally just Athens) was introduced in 1995 and was one of the first federated access management solutions in the world. It has evolved over time to embrace new technologies as they emerged such as SAML and now OIDC.
SAML federations are where you will find the customers who buy institutional subscriptions and site licences such as Universities, Colleges, Hospitals, Government departments, Corporations and Multi-Nationals.
You could connect to each subscriber individually of course, but if you're lucky that would mean hundreds or thousands of connections to manage. Federations standardise the interchange between the institution and the publisher and Keystone takes that to another level by putting all that behind one easy-to-code-for OIDC Provider that does all the hard work for you.