When you create a Revision of a Project, you are making a read-only copy of the Project and its Project files. The intent of this is to be able to compare any two Revisions against each other to view the differences between them. This functionality is most often used to then generate a "Change Order" report.
You can also create a Revision via the right-click menu, or use the keyboard shortcut Ctrl + R.
The Revision that displays in the top section of the Project Explorer is always the highest number Revision for a Project and it is the only one that is editable. Other Revisions can be viewed and opened in a read-only state via the Revisions tab in the bottom section of the Project Explorer interface: