An "open session" is a session for which opt-in messages have been sent, but not yet matching emails. It's the time interval between sending the opt-in messages and the matching messages.
During an open session, you can view the following graph in the "Session" tab of your program, updated in real time:
It gives you information about:
the starting date of the round (i.e. the date when opt-in messages have been sent)
the date when the matchs will be sent
the number of opt-ins (in the screenshot above, only 3 people have opt-in, for a total of 131 opt-in messages sent), i.e. the number of participants to the session
the number of people who declined
Even if the session is "open", you can update the session schedule and decide to send the matching message earlier or later than planned, by clicking the "Update this session schedule" button.
Be careful! By adding members while having open sessions, they may be included in these sessions according to the matching rules you've set. In the same way, these newly added members may be involved in future sessions of all your programs, unless they are not concerned by the matching rules you have set up.