Just because I dont want to pollute this thread with deep harmony stuff, i will refer you to this thread:
Roon Extension: Deep Harmony - rich feature set for Logitech Harmony - Tinkering - Roon Labs Community
The short answer, deep harmony is an extension to integrate logitech harmony hub remote control to allow Harmony to control Roon (transport, volume, some favourite and playlist shortcuts) and also allow Roon to control your Hifi devices (volume and source integration).