I just bought a new Netgear Nighthawk AC1900

> [...] Netgear Nighthawk AC1900

   "AC1900" is a speed, not a model number.  Look for "Model" on the
product label.  Current firmware version?

> What does it mean when your firmware is out of date on your device?

   Normally, it means that newer firmware is available for it.  Who said
what, exactly?

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for "firmware"?