Selections are a tool for capturing specific portions of an item that you wish to highlight or analyze further. They essentially create a cropped copy of your photo that you can then rename and add notes to.
To create a selection, click on the Select button on the photo editing toolbar. Then draw a rectangle on your photo that includes the area you wish to select.
When you have created a selection, it appears nested underneath its origin photo in the Photos pane on the lefthand side of the item view. If you click on its name there, the selection will appear highlighted in the item view.
To change the name of your selection, click on its name in the Photos pane to select it. Then click the name again to enter editable mode. You can then change the name of the selection. You can also scroll down in the metadata pane to see and edit the selection metadata.
When you have a selection highlighted, whatever notes you type in the notes field attach to that selection.