CR/LF can lead to double-spacing, notably on Mac and Unix, but at least there is a CR in there for Mac users, and there is an LF for *nix users.
If you don't know or care what this is about, please skip blithely on.
There are three differing standards for how to represent the end of a line of text. In brief, Apple Macs use the CR character. Unix and its variants use the LF character. Microsoft systems, from MS-DOS through Windows, use both together.
If you want the history behind these:
CR stands for Carriage Return, and comes from the old typewriter / teletype idea of a command to move the print head from the right of the page back to the left when it reaches the end;
LF stands for Line Feed, and comes from the old typewriter / teletype idea of a command to move the print head down a line;
CR/LF together indicate moving down a line and back to the left of the page.
The history is not relevant to today's computers in principle, but in practice they all use one of these legacy conventions, and there's nothing we can do about it but pick one.