What exactly does an editor mean when he/she says they are looking for "high concept" picture books?Basically, it means she wants a hook. She wants to be able to describe what will appeal to consumers about the book in just a sentence or two.
I, like many editors, wish more writers had a better grasp of what makes a hook and what doesn't. If writers were only sending us picture book manuscripts with hooks, we'd get a hell of a lot fewer pointless vignettes, heavy-handed lessons, nostalgic meanderings, and stories of any kind that no child will be interested in.
At the same time, some of us recognize that you can't tell writers that all you want are high concept submissions, because some of the great picture books out there are not high concept.
Skippyjon Jones, for instance. What's awesome about that book is its read-aloud quality and humor, and for clarity's sake I need to bring across that those are not high concept. Read aloud quality and humor are, indeed, hooks, but they are the kind of thing that no editor is going to accept from an author in a query letter. Because they're among the most subjective things there are.
If you can think of a snappy way to describe what's cool and fun about your manuscript, that's query letter gold. Just as long as your description doesn't include subjective descriptors like lovely, charming, funny, lyrical, wonderful, etc, etc, etc. EVERY writer thinks their writing is good, so we don't automatically believe claims of that sort. Tell us your book is about dinosaurs AND bedtime, and we'll believe you may have a hook.