The Bubble Start and End Time determines the period of time that your Bubble will remain active for. When a Bubble is active, you have the ability to shoot media directly into that Bubble using the Burst camera. You can define your Bubble Start and End time when you create a Bubble, and you always have the ability to change the Start and End time.
You will NOT be able to shoot media directly into a Bubble that is not active -- it will not show up as an option in the Bubble selector.
Just because a Bubble has ended does not mean you can't still add new media to it. You can always import media into a Bubble, regardless of whether or not it is an active Bubble.