This is reductive, but think of a telescope like a zooming lens on a typical camera. When you zoom in on an object to get a clear shot, you need to set your focus to do so. Objects that are closer or farther than where you've set your focus will be progressively more blurry and harder to make out.

What's happened here is that JWST was interested in something at a completely different distance, but caught a blurry image of something much closer.