What does Visual Studio mean by normalize inconsistent line endings?

Question

Visual Studio occasionally tells me:

The line endings in the following files are not consistent. Do you want to normalize the line endings?

Answer 1

The file you are editing has been edited with some other editor that does not use the same line endings, resulting in a file with mixed line endings.

The ASCII characters in use for line endings are:

CR, Carriage Return
LF, Line Feed

Windows = CRLF
Mac OS 9 or earlier = CR
Unix = LF