Clicking ONCE on the C drive icon shows you that (in this case), about a third of the drive has data on it.

GB stands for "gigabyte" (pronounced GIG-a-bite with hard G's), which is "roughly" a billion bytes, or a thousand megabytes.. A gigabyte is two to the 30th power.

Summary:
8 bits = 1 byte
1024 bytes = 1 kilobyte (KB)
1024 KB = 1 megabyte (MB)
1024 MB = 1 gigabyte (GB)