Line editing is about expression. Do you have chop on the waters of your writing, where every sentence seems to be under 12 words? Or is there a setup sentence or a summary sentence that your line editor might add that would establish a place for a passage that follows? If there’s a place for a brief bit of clarification or a dash of exposition, a line editor will suggest it, or just add it depending on the author’s preference.
I'm amid the revisions after a line edit. My editor suggested the line "He did not return my good humor." Well-placed, that one, and right in keeping with my tone. The good line editors can match your syntax, when needed. Line editing is the most collaborative of the edits, because you get a second writer adding to and eliminating passages that need help, or need to go.
By contrast, copyediting checks punctuation and grammar, usage and spelling, how your tenses agree with one another, consistency of usage, and some fundamental fact-checking: names, locations, and trademarks are among the targets for a copyeditor's revisions.
Line editing won't change a story. That's for development. But a good line edit can be the "just right" among the three bears of editing: develop, line, and copy.