nerdfont.csv is a generated CSV that includes all of the Nerd Font icons and their description, as found on Nerd Font's Cheat Sheet website.
nerdfont.txt is a more readable version of that file
You will need to open the files with an editor that has a Nerd Font installed for.
A simple icon picker can be built with rofi (or dmenu) and the provided nerdfont.txt file. This picker will copy the icon to the clipboard (requires xsel installed)
cat "nerdfont.txt" | rofi -dmenu -i | awk '{print $1}' | xsel -i -b
In wayland with fuzzel and wl-copy you can issue
cat nerdfont.csv | fuzzel -d | gawk -F, '{printf $3}' | wl-copy
The prefered method to update the files is now through a copy of webfont.css and the newer updateScript.sh bash script. However, the older updateScript.js is still available (see it's content for uage instructions. updateScript.sh takes exactly one parameter: a path to webfont.css. If this is empty, the script will attempt to use curl or wget to download the file.