In the simplest terms, C and D are arbitrary names assigned to either physically separate or logically separate disk areas where things can be written.
All local drives in Windows use drive letters. By tradition, A and B are reserved for things like floppy disks even if your system no longer uses them. The first hard drive traditionally was usually called C because A and B were already in use. The next drive slot would be D, and so on.
When disks got bigger and started to be partitioned, Windows had advanced enough to allow for recognizing the partition as though it were a separate physical drive. It doesn't matter that a disk partition is merely a way of subdividing disk addresses into multiple ranges that act like different disks even if they aren't different disks. So now, C and D can be either two physically separate hard drives or two partitions on a single hard drive. To everything except the disk device driver, it makes no difference in the way you use them.
Be VERY careful of what you move from your C drive to your D drive, and it probably would not hurt to do a disk cleanup to remove old download content. I worked with the U.S. Government as a mainframe administrator and I can tell you, the biggest problem I ever had was users who would not erase old, useless files to make room for newer files. Learn cleanliness and make your resources last a lot longer.
When cleaning, files in any directory path that includes either /SYSTEM or /SYSTEM32 or /PROGRAM FILES are not good choices for cleanup.