The path describes how you would get from the main or root directory to the file. The main or root directory of a disk is designated with a backslash symbol (\). Therefore, the main or root drive directory is designated as C:\ and the A drive root directory is designated as A:\. Sometimes DOS lets you abbreviate.
So at times, you could designate the root C drive as simply C: and the root drive as simply A:, but until you understand how to use these abbreviations, it is much safer to use the actual names (A:\ or C:\) when referring to a directory.
The path traversed to get to NOTES.TXT is C:\_DAT.