How can I create a .jar file?

[Albert Bence]

"If you need a jar file (e.g. for cordova) run gradle task createFullJarRelease and use full.jar"
This is the only description I find here.

It's my first time working on an Android project. I don't even know what a gradle task is, let alone how to run it. I tried making a .jar file using windows cmd java compiler, but it didn't work.

I know I'm probably missing the obvious, but please, someone explain me. I wasted two days of my life trying to create one.