They are text files.
They change the way textures are displayed.
Notepad is a good option.
You direct the shader to the texture you want, then put the shader in the shaders folder. This makes more sense when you understand how shaders work, which takes some practice.
Will they help with light-emitting textures?
They don't help
with light-emitting textures, they create
light emitting textures.
Now I gather you're going to have some more specific questions, now that I answered the basics, so fire away.