The feature is pretty self-explanatory. When enabled, any new members that register will be placed into the "Validation" group rather than the typical "Member" group. This will allow you to set special permissions for members that have registered, but have not yet been given the go-ahead from an admin.
- Login to the ACP
- Browse to Members->Member Validation
- Enable it!
Let me know if you have any questions or suggestions for improvement.