https://www.cnet.com/news/apple-will-warn-you-if-your-old-32-bit-mac-apps-wont-work-well-as-it-moves-to-64-bit-with-macos-high-sierra/

If you have the latest version of Apple's MacOS software downloaded, you'll get a warning when you open older apps that run on 32-bit technology. 

Apple has been pushing developers to update their apps to 64-bit technology. That enables faster apps that can juggle large amounts of data more efficiently than the 32-bit processors common in older computers and mobile devices. Apple's modern, underlying technology for its computers -- such as its Metal graphics tool -- work only with 64-bit apps, and the processors in its Macs have been 64-bit for years.