Virginia,
You are correct. We lack proper documentation/explanation of Baselining in our User Manual. I will be sure to escalate this as a documentation request.
In the meantime, a quick overview of Baseline is as such...
Lets say you have a standard flow that your projects must go through. Whether they are similar Milestones and Tasks, Dates, Work and Duration or Budget information, this information can be saved as a Baseline similar to saving as a Template. This is similar to a Template but the main difference is that you will not use it when creating new Projects.
What you will be able to do is compare existing Projects (ongoing for example) with your Baseline to make sure that Projects are staying within their schedules. You can compare. you can view Projects vs. Baseline in Roadmap and report on this as well to gain clarity on how your Projects are being handled per requirements.
Hope this information helps.
Sincerely,
Boris