What is a "Drive Letter" on Windows?
A drive letter is a letter assigned to a "drive" on Windows that uniquely identifies that drive in filepaths. For example, in the filepath C:\Windows, C is the drive letter. Because drive letters come before a colon (:) in filepaths, typically you'll spell them such, e.g. the "C: drive" (not the "C drive").
By default, Windows is installed in the C: drive. If a second drive is found, e.g. if a USB flash drive is plugged into the computer, it will get the D: letter, then E:, then F:, and so on.
C: drive.