The current Version will show you in which version of the planning cycle you are. & with it sync with all your module in the model. It means when you open the module it will show the current version as the first version by default.
1. Let's say you have selected as budget as your current version
When you open the module, first version will show you as budget
2. Now I have select Forecast as my current version
You can select only one version as your current version.
The current version check box serves as the base for the 'CURRENTVERSION' function. It gives you an easy way to refer to the currently used version in the system to be referenced in formulas. However you can also leave this checkbox blank if not required.