Let's say you use tables a lot, and you want the text size bigger so you can see the text from a long distance. To do so, you use zoom level. You can use zoom on your dashboard or on the TV.
#1: Here's what the dashboard looks without using zoom level.
#2: Here's what the dashboard looks like when using zoom level; the widgets on the dashboard will automatically adjust, and the text size gets bigger.
Using zoom level on the TV screen:
Exit full screen, and next to the browser there is a zoom icon. Click here, and zoom in on the dashboard until you reach a zoom level suitable for your screen and the layout on the dashboard/slideshow.
Using zoom level on the dashboard:
Go to your dashboard, and use cmd + to zoom in on the dashboard. You might need to change the size of the widgets on the dashboard to visualize all.