This endpoint lets you generate a connection token to authenticate a user without having to indicate its credentials. To be able to generate a connection token for a user, you need to have WRITE access on that user and that user cannot have PROVIDER role. A connection token is valid only once and has a time to live of maximum one day.
The authentication system on Partoo API is using API Key that should be put in the header of the request (the name of the header is x-APIKey). An api_key is linked to a user. This user's role will give you different access level to the API features.