Kismet
Kismet is the Visual Coding Software included in Unreal Editor 4. It greatly enchances in-editor coding that was available in previous Unreal Editors.
Basic Concept
The basic concept is similar to previous UEd's Scripted Triggers but hugely more complicated and you dont need huge numbers of actors to follow multiple sequences of code!
The visual aspect has a flowchart style where individual components can be linked together in various ways.