macOS Mojave update - what was updated?
Apple's Downloads

Apple provide release notes for each update in the macOS Downloads section of their support site.

As an example, macOS Mojave 10.14.2 Update, states:


  The macOS Mojave 10.14.2 update improves the stability, compatibility and security of your Mac, and is recommended for all users.
  
  This update:
  
  
  Adds RTT (real-time text) support for Wi-Fi calling
  Adds a menu item to News for opening a story in Safari
  Resolves an issue that may prevent iTunes from playing media to third- party AirPlay speakers.
  
  
  For more detailed information about this update, please visit: https://support.apple.com/kb/HT209149 
  
  For detailed information about the security content of this update, please visit: https://support.apple.com/kb/HT201222


Follow the detailed information links for specific changes that are included.