Verbose mode

Verbose mode is a boot mode on computers running Mac OS X or macOS that replaces the graphical startup screen with technical information that can be useful for troubleshooting or development by advanced users who are familiar with Unix. It can be accessed by holding  ⌘ + V upon startup.

Articles

 * How to boot your Mac in Verbose Mode by Christian Zibreg at iDownloadBlog (2015-08-17)
 * macOS Hidden Treasures: 15 Startup Key Combinations by Josh Centers at TidBITS (2016-09-01)