Hi NDR,
The system will automatically mark your lesson as complete when you view it, yes. You can still manually click the button to mark it as incomplete / complete, depending on what you want.
We understand that learning guitar is different for everyone, and one person may feel comfortable enough to move onto the next lesson and leave it as "complete." That's just the way we have it set up right now and again, feel free to mark lessons as incomplete, as you feel fit. To be honest, it's just really difficult to determine wether someone that is watching a lesson actually "completed" or understands the concept in the video fully, etc.
It is an imperfect system but we believe it works well for most people and does a good job of keeping track everything that you've gone through. For more info on how to use the system, check out this page: https://support.guitartricks.com/hc/en-us/articles/216287387-How-do-I-use-the-Progress-Tracking-feature-
-Billy