beginner: how do I create a custom subject?

Greetings,
DIVI (elegant themes) support themes suggested I use this plugin if I want to change the subject – so when I receive an email sent to me from someone who filled out the form, I’ll see the custom subject in my email box – instead of the name of my website.

In the default form that comes with this plugin, I see this code in the ‘subject’ settings: [_site_title] “[your-subject]”
Should I replace _site_title with the subject I want to use, or should I replace your_subject?

Or – should I write something in between these two bracketed areas?

I know this is a stupid question. If anyone know the answer – I greatly appreciate it.