i keep hearing that you can do something with activities, in game. would somebody please explain?

All (excluding Tutorial Mission, I think) activities (like skirmish defense, one man army, ect.) are now run using Lua. For example, in the past, Skirmish was pretty much hardcoded and couldn't be changed. Now though, it's 100% scripted, like everything else. This gives you more control over what you want your scene to do when you script it.