The Lutris runtime is a set of shared libs used by the Lutris client in order to avoid depending on what's installed on the system, thus providing support for a large quantity of games out of the box.
The scripts provided build the Steam runtime (used as a base), then install the packages for extra libs used by Lutris. You must be running Ubuntu 14.04. Then, with ldconfig -p, it looks for the path of the required .so paths and copies them into the runtime directory.
Extra libraries that are not available in Ubuntu are included directly in the repo. Here's a list of these libs and their source: