Can you elaborate how 'Ctrl-x f' looks up a file name?

I tried 'Ctrl-x' but it decrements the integer under the cursor by 1. I tried 'f' but it starts a new motion command to search the next occurrence of the character following it to the right. Example: 'fx' will place the cursor on the next occurrence of 'x' to the right.

How can I use 'Ctrl-x f' to look up a file name?

In addition to being an insert mode command as jshholland already noted, it is Ctrl-x Ctrl-f, not Ctrl-x f.

Here is a description of it and a few other insert mode commands: https://georgebrock.github.io/talks/vim-completion/

Fascinating. These ctrl-anything is like a dangerous, crippled emacs lurking in a dark corner of my beautiful vim, ready to terrorize with keyboard coords over modes and commands ;)

I've bound some complete-plug-in to tab, but that's pretty much all I do in insert-mode command-wise.

It's an insert-mode binding, built in by default.