CTOT = Calculated Take-Off Time
For the purposes of airports, en-route traffic management and arrival at the other end this gives everyone a chance to regeulate the flow of traffic to keep things manageable (in theory).
as i understand it, the 'Slot' is 5 minutes before and 10 minutes after the CTOT. Therefore if you have a slot the earliest airborne time is five minutes before and latest ten minutes after. If you get to the holding point early they will leave you there and if you are looking for push/taxi close to the slot they may tell you that it is too late and you need to apply for another slot.
As far as flight plan expiry...can't remember all i know it's happened to me a few times
Hope that helps