What is the meant by "A high standard of illustration (both line and photo) is an editorial"? A high standard of illustration (both line and photo) is an editorial
  priority.


What is meant by "line" here? I am not getting this into my mind. The sentence above is taken from the "Instructions to author" part of a journal.
"line" means "line art", and could be anything from pen drawings to graphs (plots) or other illustrations drawn with a thin pen or its electronic equivalent.

The reason line art is often listed separately from photos is that it requires a higher resolution image and usually also a different file format for high quality output.