Location previews are a way to quickly inspect the recent media tagged at a location and bookmarking them without leaving your current view. These are similar to the long-press actions or 3D Touch behaviours found on the mobile Instagram apps.
To view a location preview:
Long-press on any location name in the feed, or details view.
To close a location preview, press the ⎋ (ESC) key, or click outside of the preview.