A computer is a machine that follows a list of instructions called a program. An Android device is a computer and an app is a program.
An app must be written in a programming language such as Java because devices do not yet reliably understand human languages such as English. In fact, the device would become hopelessly confused if we wrote a sentence in English in the middle of the instructions of an app.
But we often want to do just that, to provide an explanation of the app addressed to human beings. This explanation is called a comment, and must be surrounded by comment delimiters: punctuation marks that tell the device not to read the enclosed text.
In a Java file, each comment surrounded by the special delimiters /** and */ can be automatically copied into a separate file by a tool called Javadoc. These comments are therefore called Javadoc comments, and the resulting file of documentation is a Javadoc file. If we write a Javadoc comment in front of each class and method that we create in the Java file, the Javadoc file will provide a summary of the Java file for the human reader. To encourage us to write these comments, Android Studio can write part of each one for us.