There’s really nothing much to explain. The header is the photo across the top of the page. I could select one of a handful of stock photos to take this space, or use one of my own. I choose the second option. At first, I only tried to look for an attractive picture that would add a bit of interest to a word-heavy blog. I soon learned that only certain photographs look good in a long, narrow, horizontal space. I quickly started looking for, and seeing without even trying, lines: elements of buildings, gardens, nature, people, and more. I have come to relish taking header (and other) shots with my decent but decidedly not-professional camera.
Note the tab at the top of the page called Header Notes. This is where you can click to read a description, background notes and if you’re lucky, a funny, clever caption for the header photo.