:so %, short for
:source %, is:
:help :source command, which is used to "source" vimscript files,
%, which is a shortcut for "the current file".
Therefore, it means "source the current file" or, at a lower level, "execute all the Ex commands from the current file".
If the current file is not a vimscript file, say you opened a
.yaml file, then "source the current file" is a pointless thing to ask Vim to do because it is very unlikely to contain any vimscript.
From Vim, the proper way to source your
vimrc is to use the
$MYVIMRC environment variable (that is only set IN Vim):
NOTE: As hinted in the comments, not everything you can put in your
vimrc can be expected to have an immediate effect upon sourcing and settings can come from other places than your
vimrc anyway, so the command above is not a silver bullet. Starting a new session is a much better solution.
NOTE: Editing your
vimrc in a different editor makes no sense. Doing it in Vim gives you proper syntax highlighting, completion, and direct access to the documentation via
:help K. And FWIW, it also allows you to do things like
:w|so % while you are editing your