I don't know about FRB, but I can summarise the event handlers for you.
Publishers raise events when interesting things happen, and subscribers listen for the events and react accordingly. The Geom in this case is the publisher, and one of your game objects like an actor is the subscriber, and the interesting event is
Getting more specific, a geom object exposes an event called "OnCollision" with arguments (Geom geometry1, Geom geometry2, ContactList contactList). If you create a method with that signature you can subscribe to the event, which means
your method is "listening" for the event and will fire when the geom object decides to raise the event. So pretend you've got a method called MyCollisionMethod, like this:
protected void MyCollisionMethod(Geom geometry1, Geom geometry2, ContactList contactList)
When you create your geom object, once you've created it you add this to take out your subscription:
geom.OnCollision += MyCollisionMethod;
This attaches your MyCollisionMethod method to the event OnCollision. It is important to then unsubscribe to the event when you are finished needing it, so you add this:
geom.OnCollision -= MyCollisionMethod;
The reason is that a subscription keeps a subscriber alive, and it wont be eligible for GC until the publisher has gone. Basically just add that -= unsubscription line when you are getting rid of the object that was listening to the event.