What process happens when there is a system optimization?

**My question is the following:** Not long ago I was moving apk, data and .dex files to the directories / data, and they did not show the applications I had manually installed, if not until I rebooted on the device and a message appeared saying "optimize the system" from the moment I was curious about this. My purpose is to learn what happens in this process and to know how to be able to repeat it manually. If you know anything about this, I'll be happy to hear your comments and answers.