Rosetta is a translation process that allows users to run apps that contain
x86_64instructions on Apple silicon. Rosetta is meant to ease the transition to Apple silicon, giving you time to create a universal binary for your app. It is not a substitute for creating a native version of your app.
To the user, Rosetta is mostly transparent. If an executable contains only Intel instructions, macOS automatically launches Rosetta and begins the translation process. When translation finishes, the system launches the translated executable in place of the original. However, the translation process takes time, so users might perceive that translated apps launch or run more slowly at times.
A short overview of Rosetta 2, the translation layer that allows 64bit x86 applications to run on the upcoming ARM-based Macs.