"commenting a line" means to change it so it's taken as a comment in the language, so it is ignored. You could also delete it altogether. But changing it to a comment keeps it in there in case later on you want to uncomment it.
In shells, comments are prefixed with a # character. So commenting the
source <(ng completion script)
line means changing it to:
# source <(ng completion script)
That means however that that line which sounds like it's meant to configure completion for some software will not be run.
So instead, you can instead fix it so it works properly.
You'll want to double check the instructions for the software that asked you do add those lines if you haven't missed some step.
Here, it seems that thing expects compinit (which defines the compdef function among other things) has been run beforehand.
Maybe the call to compinit is further down in your ~/.zshrc, in which case you'll want to move that line below it, like at the end of your ~/.zshrc.
Or maybe it expects you to have enabled some third-party plugin system for zsh such as oh-my-zsh, or to have run compinstall, etc.
To run compinstall (to configure completion), run:
autoload compinstall
compinstall
That will add lines to ~/.zshrc including compinit. You'll still need to move that source... line below those added lines.