Mailing lists are used in a lot of open source projects. They are essentially a server you send email to and that broadcasts the email on to a load of subscribers. This simple model has a few functions:
- Few to many/announcements. It's common that a release mailing list will have a few people who can broadcast email about updates and releases. People who need to know about these subscribe.
- Devel/discussion. These are more like a forum where developers are free to table and talk over development topics.
- Support. Identical in method to a discussion list but set aside for users to ask support questions, like here.
How you use them is largely up to you. You can read Ubuntu's without anything more than a browser as they are all publicly archived. If you want to subscribe (so that new messages are delivered into your inbox) you can from the list's listinfo page.
If you want to be able to post, most lists will require that you subscribe first. For some subjects this also requires that a moderator authorises you to send email. But sending the messages after that point is no harder than sending an email to the list's email address.