The runtime component means that the system can run inside a game or other real-time application, so that the world can react to gameplay or geometry changes. The PCG tools can also be used for linear content requiring substantial numbers of assets, such as large architectural projects or film scenes.This is an Experimental feature that will be further developed over future releases.
Substrate
This release also introduces Substrate, a new way of authoring materials that gives you more control over the look and feel of objects used in real-time applications, such as games, and for linear content creation.
When enabled, it replaces the fixed suite of shading models with a more expressive and modular multi-lobe framework that provides a greater range of surface appearances and a wider parameter space from which to work. It is especially powerful for describing layered looks, for example “liquid on metal” or “dust on clear coat.”
Source: Unreal Engine Blog