What does C$ mean when entered in Windows/Run while accessing remote drive?

If I enter in the Windows Run "\_some_ip\c$" I go the remote C: drive. What does the C$ stand for and why isn't it C: ?

Shares with a $ are hidden shares (also called "Administrative shares" as mentioned by Diskilla) and thus not visible when requesting the share list of a device.

There are several default hidden shares active by default on your system:

• One for every drive available:
  • (A$)
  • C$
  • (D$)
  • etc.
• To C:\WINDOWS: ADMIN$
• Printer sharing: print$
• IPC (Inter-process communication) interface: IPC$

For more information about (creating and removing) hidden shares, check out the Microsoft Knowledge base on this topic.

If you right click "(My) Computer" -> "Manage" -> Select "Shared Folders" on the left -> Go to "Shares", you'll see something like this (but in your language of course). This one is in Dutch: