The Platform API is the Xkit API for use by a system which can keep secrets, like a backend server. It has full access to all users that have been provisioned for your Platform with Xkit.
With the Platform API, you can generate tokens for your users to use to connect with Xkit as well as get access to all of the access tokens for your users' connections to 3rd party applications.
To use the Platform API, you need to have an Xkit developer account, so if you don't already, go ahead and sign up - it's free to get started.