TL;DR:
It's alive! You can now accept donations!
The Details
Publishing your campaign means that your donation link is now active and can accept donations. There are some settings of your campaign, such as the Title and Campaign URL, that cannot be changed once your campaign has been published, so please make sure you are ready to publish your campaign before you actually publish it.
Some settings, such as the donation goal amount or adding incentives, can be changed at any time and will be reflected on your campaign page immediately after saving (but will sometimes require a refresh on the user side).
As long as your campaign has not accepted donations, you can unpublish your campaign. Once a donation has been made, you can no longer unpublish.