Zoom/Expand part of my Mac's screen
You can use the built-in zoom feature to zoom in on a part of the screen on your Mac. 

(Note: these instructions are adapted for El Capitan but also work with Sierra and as far back as Lion, though certain menus/buttons may be labeled differently).


Go to System Preferences
Navigate to the Accessibility Settings menu
Choose Zoom from the pane on the left.
Enable Use keyboard shortcuts to zoom.
Enable Smooth images (for better quality with images)
Enable Zoom follows the keyboard focus.
Ensure your Zoom settings match these:





Then, choose between Picture-in-picture and Fullscreen for the Zoom Style; see the pictures below for help choosing.


Zoom in picture-in-picture mode:



Zoom in fullscreen mode:



After turning on Zoom, you can control it with keyboard shortcuts, as mentioned by Paul Zagoridis:

Command+Option+8 toggles zoom on/off. 

Command+Option+= zooms in. 

Command+Option+- zooms out. 

Source + more info: http://basics4mac.com/article.php/magnify

Reply below for any additional help or if you have questions; I hope this helps!