Wilfred Hughes has tweeted another great Emacs Command of the Day:
Want to change the line ending style between DOS and Unix line endings?
You can use set-buffer-file-coding-system (bound to C-x RET f) or just click in the mode line!
H/T https://t.co/7roHvPnxeh pic.twitter.com/lzSLqPKC4n
— Ⓔ Command of the Day (@ecotd) June 29, 2018
You can follow the link in the tweet for a bit more information but the TL;DR is that unless you have special needs, you should be seeing U: as the first two characters of your mode line. The U means that the encoding is UTF-8 and the : means that lines are terminated in the Unix way with a newline character.
You can click on the : to cycle through the other possibilities but these days you probably want either : or DOS.
And who knew Ctrl+x Return was a valid command prefix? The completions all have to do with setting various encoding options.