What happens when I mark a partition as active?

The partition that has Windows on it is already marked active.

If you mark another partition active, it will mark all other partitions as not active. If you reboot your computer with the Windows partition mark not active, it will not boot into Windows; it will probably say OS not found.

Unless you feel comfortable with the partitioning, formatting and installing files, I suggest that you leave the modifications to someone with more experience.