I agree it is quite confusing and not particularly well documented.
This is my understanding:
A series is basically a set of releases. Normally you have a main development branch associated with a series called 'trunk' or something similar. You can have other series such as 'stable' with their own sets of releases. Confusingly, series can share or have separate bzr branches - I'm not sure what the best thing to do in this respect is.
Within a series, you have milestones. It seems you can only set one milestone at a time - you need to release your first milestone in order to set your second. Milestones will probably be your version numbers eg. 0.1, 0.2.
When you click 'Release now' to release a milestone, you will be able to upload files for download that correspond to that release.
The main points to this:
- Branches are completely separate lines of development.
- Series are parallel sets of releases.
- Milestones are future releases and work in a linear fashion within one series.
- Releases are past milestones that can have downloads associated with them.
(sorry for the circular definition here but thats how it works).