When does it show a FB user active?

I'm just curious if when Facebook shows a person was (active 2 hours ago) in a message if that means they just opened/were scrolling through the app or if this means they opened/responded to their messages?
All being seen on iPhones

It's means they opened the app 2 hours ago.

The 2 Hours Ago is When they Were Last Active

You can only be active on facebook when you log in but if such happens it could be from the internet connection

They opened the app 2 hours ago.