It quite literally means that the airline is planning on launching an international route, but can't legally operate the flight until various government approvals are received. In international flights, the originating and destination countries must agree to grant an airline the right to operate a route (often including the departure and arrival time slots as well).
You also ask:
How does the likelihood that such flight will occur compared to other flights along the same route?
That's for you to decide; for me, if the airline is a major one (e.g., BA, AA, CX, JL), I'd figure the likelihood is quite high and would and have booked tickets before final approval.
What should I expect if it does not occur, compared to the cancellation of a flight without that note?
You should be treated the same, meaning that if you have a paid ticket, the airline will offer you a full refund or reaccommodation on another flight.