They are those sessions where more than one user is riding at the same time, and to which you can join and pedal directly.
Once the session is selected and before it starts, you will see the session's specs: distance, elevation, AVG slope, and the time since the session started. If you decide to join, you can do so from the start or from the position of the last rider.
You can also see who the other riders are and their level by clicking on the "Cyclists" tab.
When you access a BKOOL Cycling route, you will see if there are already sessions started by other users. You will be shown if the session is in progress and the number of users that are in it. You will be allowed to join a session during the Warm-up period and if there is a group outing organised by BKOOL within the next hour, you can sign up for it.