If you regularly update your apps or download them only from the App Store, you'll most likely be fine. As part of the transition, Apple last year started requiring all apps in the App Store to use the 64-bit format. The move to exclusively using 64-bit apps in MacOS Catalina has been in the works for years, and Apple has been helping developers transition their 32-bit apps over to the 64-bit format. Overall, this is a good thing because it means that apps will run faster and access more memory.
MacOS Catalina is here, and with the update comes the transition from 32-bit apps to 64-bit apps. Make sure you check to see which apps won't work before you update to MacOS Catalina.
