Compiling a file in Java means literally transforming the code in bytes and bytes and bytecode. You make from a .java file a .class file.
Sometimes are people using the term “compile” as synonym of run your code in your development environment like Eclipse/intelliJ IDEA/NetBeans
(example if you run file in Eclipse)
But most of the time especially for school assignments they mean that you have to transform your code into bytes and bytecode in order to use it efficiently on different devices.