A Jupyter Notebook can either be in edit mode or command mode. When in edit mode, the contents of a cell can be changed; the cell gets highlighted in green, as shown in the following screenshot: 

You will then notice a pencil icon appear in the top right corner:

Edit mode can be entered by clicking on a cell or by pressing Enter on the keyboard.

The Esc key helps us switch from edit mode to command mode. This can also be done by clicking anywhere outside a cell. A gray boundary around the cell with a blue margin on the left indicates command mode:

Command mode allows us to edit the entire notebook, while edit mode serves more like a text editor. The Enter key helps us enter edit mode when we're in command mode. Several keyboard shortcuts are available in edit mode and command mode. Command mode is mapped to a higher number of shortcuts than edit mode:

The keyboard shortcuts in the preceding screenshots help us navigate Jupyter Notebook.