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.

Since Android devices do not yet reliably understand human language, apps must be written in a simpler language such as Java. The instructions written in Java are called code and can be understood by the device.

But it can be very tedious to write an app in the tiny steps that are describable in Java, so we first sketch out the app in a human language. This paraphrase is called pseudocode, and can be understood only by a human being.