To ensure that the apps you purchase are as advanced as the Mac you run them on, all future Mac software will eventually be required to be 64-bit.
Apple began the transition to 64-bit hardware and software technology for Mac over a decade ago, and is working with developers to transition their apps to 64-bit. At our Worldwide Developers Conference in 2017, Apple informed developers that macOS High Sierra would be the last version of macOS to run 32-bit apps without compromise.
While developers optimize their apps for 64-bit compatibility, Apple is notifying customers when they are using an app based on 32-bit technology. This is done via a one-time alert that appears when you launch a 32-bit app.