The Mac has been shipping with 64-bit processors since before the Intel transition and macOS, née OS X, has supported 64-bit for years. Modern technologies, like Metal, require 64-bit. One day, macOS will as well. Apple hasn’t said what day that will be, at least not yet, but it’s continuing its long, slow march in that general direction.
At WWDC 2018, Apple noted that macOS Mojave will be the last Mac operating system to support 32-bit apps. By fall of 2019, and possibly sooner, any 32-bit apps still on your Mac will no longer work.
Apple has been notifying Mac users since April 2018. If you open an app on your Mac and get an alert that an app isn’t optimized, there are a few things you can do.