What is sIBL?
sIBL is short for "Smart IBL" . It's a standard describing all information and files needed to provide fast and easy Image Based Lighting Setup.
sIBL-Gui is a free application for organizing your HDR images and setting up your scenes with a single click.
But you are not required to use sIBL-GUI to use these files. You can also set up you scenes as usual.
Using HDRi in Blender with Cycles
To use HDR images in Blender you need to assign it as a world texture. This can be done by selecting Environment Texture
as the color input, and then opening the chosen HDR image.
The node setup should then be a Environment Texture connected to the Background shader node, which in turn should be connected to the World output.