How do I solve this error message on my ghost site?

Recently I’ve come across this message when logging onto my site


Now, I’m not an Html expert at ALL.
How can I solve this?
And could the person that answer this, explain like it was for a 5 year old?

The newest version of Ghost displays more validation of the theme than the old version used to.

None of that is critical to change. You can choose to ignore it, if everything is working well. Otherwise, you can open that amp.hbs file and edit it to address each point.

How do I access the amb.hbs file?

You need to download and unzip the theme. Directions are in the tutorial section.