Essential services are those in the Core Services layer. When that option is checked, only the incoming connections to the below services will be allowed:
The technologies in the Core Services layer are called core services because they provide essential services to apps but have no direct bearing on the app’s user interface.
Some of the notable core services are
- accounts & Social media integration
- identity service
- time machine
- keychain services
- iCloud storage
- file coordination
- bundles and packages
- security services
- address book
And from http://www.macworld.com/article/1131116/firewall.html#configure
The second option, “Allow only essential services,” will block anything except a few default services that support networking, such as Bonjour. Only use this option if you want to block everything; this will prevent any file sharing, remote access, or other services activated elsewhere on your Mac.
You can switch your firewall settings and run
nettop in Terminal to monitor the network activity of each process that is running.