xkit.js is a browser-based Javascript library for working with the Xkit in the context of one of your end users (a "Platform User", i.e. one of your applicatin's users which you have provisioned on Xkit).
With the xkit.js you can log users into Xkit, retrieve their access tokens for 3rd party applications, and manage their connections to 3rd party applications (although you can use the xkit-catalog.js to do most of the heavy-lifting for you).
To use xkit.js, you need to have an Xkit developer account with a provisioned platform user , so if you don't already, go ahead and sign up - it's free to get started.