This shortcut zooms in on the current worksheet, making items larger and easier to read.
Note: you can also use Control + mouse scroll wheel to zoom in and out on both Windows and Mac.
On Windows, the shortcut appears to be new with Excel 2016, and changes increases the zoom level shown in the lower right of the worksheet by 15% each time Control + is used. Use Control - to zoom out by 15%.
On the Mac, there is no native keyboard shortcut to zoom in Excel, as far as we know. However you can use the following system level shortcuts to zoom in and zoom out:
Zoom in - Option Command +
Zoom out - Option Command -
Toggle zoom - Option Command 8
You can find these at System Preferences > Accessibility > Zoom