Black bar on right side of screen

Hi
I am very new to i3, so just experimenting an no expert on nothing
But I had the same problem, and it was intermittent, and I managed to solve it by moving the monitor line to the end of the config file and putting a sleep 2 in it.

I think it was trying to do the dual monitors whilst also doing the Nitrogen wallpaper, or something similar.

It would always work if I manually ran the same monitor.sh or the same xrandr command line after boot

What I dont understand is how this file is ‘read’ in terms of what is done when, is it just sequential in terms of the exec lines?
I read that it was ‘asynchronous’
What does this mean, in this context?
Not just simply that they are not in sync?