What Is an IES File?
A file with the IES file extension is an IES Photometric file that stands for Illuminating Engineering Society. They are plain text files that contain data on light for architectural programs that can simulate light.

Lighting manufacturers might publish IES files to describe how various structures are affected by their product. The program using the file can interpret it to understand how to display the correct lighting patterns on things like roads and buildings.

photometricviewer.com – IESviewer (Andrey Legotin)
The photometric data files (IES, LDT) commonly used by lighting fixture manufacturers to distribute the photometric information about their products.

EASY BUTTON Understanding IES (Illuminating Engineering Society)
An IES file is basically the measurement of distribution of light (intensity) stored in ASCII format.

IES Light Profiles, An overview of setting up and using IES textures with lights.

CNDL, An open source IES lighting creation tool
github.com/nickmcdonald/ies-generator, An easy to use program that generates custom IES files

IES lights with VRay (YouTube)