During Apple’s Platform State of the Union keynote at WWDC, the company informed developers that macOS High Sierra will be the last release of macOS that will support 32-bit apps “without compromises”. This means that starting in January 2018, new apps that are submitted to the Mac App Store must be of the 64-bit variety, and that all existing apps in the App Store will have to be updated to 64-bit by June 2018 if they wish for their app updates to be approved.
For the end-user you probably won’t notice anything, at least on the surface, so this is more for developers. To Apple’s credit, this isn’t a last minute decision that they’re springing onto developers. The company has given iOS developers quite a while and has sent out various warnings and reminders to get them to get their apps updated to 64-bit, a courtesy which Apple has stated that they plan to extend to macOS developers as well.