vim-airline: what is "! trailing[1]"

That means you have a trailing whitespace on the first line ([1]).

You can add to your .vimrc the following:

set list          " Display unprintable characters f12 - switches
set listchars=tab:•\ ,trail:•,extends:»,precedes:« " Unprintable chars mapping

That'll display whitespace chars. You can toggle it with :set invlist.

Airline is telling you that on line 1 you have trailing whitespace, which is usually something you want to get rid of.

So go to the line and delete it (1G$gelD).

It’s a good feature, but you can turn it on/off with:

:AirlineToggleWhitespace

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