How do you add a background landscape?

As a beginner, I’m not sure what the proper terms are for this. I have a flat grassy field in a level and I want to make it look like it goes on all the way to the horizon. I don’t want to add too much components to the landscape, I was thinking more like an image, the player will not be walking on it. I just don’t want them to be able to see the edge of the landscape like the big square it is, I want it to look like the horizon.
Thank you.

Not a problem to be a beginner. But , I am not able to help you as I am not able to understand what you are trying to say. Read this if it helps.