Using Natural Framing in photography is a way to add to the composition of the photograph by focusing the attention of the viewer on a specific aspect of the photograph. I suppose you could say it is “obvious lines” that draw the viewer into the photo.
From Digital Photography School:
“Tips for using Natural Framing to Improve your Composition” is written by Anthony Epes and does a great job of explaining how to use this for composition: Natural framing is when you use an element within your image to frame the subject. This draws the eye into the photo and highlights the actual subject.” This article is a very good place to start.
“How to Use Framing for More Effective Composition” by Jeremy Schatz
From Photography Life:
“Framing Subjects with Natural Elements” is written by a photographer who does primarily weddings and portraits but Lola Elise has some very good suggestions on how to use nature and doorways, windows, arches, etc.