You can use the preview window to examine a single or set of photos. You can use the preview window in combination with keyboard shortcuts to quickly cull and rate photos. In addition, the preview window displays a concentration of metadata and other information about the photo. 

The preview window appears when you hit the spacebar in a contact sheet,  select the magnifying glass button on a thumbnail in a contact sheet or, with at least one thumbnail selected, use the keyboard shortcut of command-R on macOS or Control-R on Windows

The preview window, by default, displays two panes: The thumbnail strip and the info pane. Both are resizable by dragging. The thumbnail strip shows the photos adjacent to the preview photo. Use the buttons to display all photos in the contact sheet or just the photo or photos you currently have selected.  

The info pane shows information about the photo, including Info, Crop, Zoom, Histogram, and Highlights & Shadows. Use the triangle to expand or hide information.

The control buttons allow you to: navigate to the previous and next photo in the thumbnail strip, remove or add photos from the set, rotate photos, open the Metadata (IPTC) Information tool, open the photo in an external editor, delete the photo, view single or 2-Up images, change the thumbnail strip orientation (vertical or horizontal), and hide or restore the panes, and toggle color management on and off. 

Hover above a button to find out what it does. You can also use keyboard shortcuts for all of these actions.