Chances are that you will work on projects with more with one
Java files.
Plus you may be using resources from third party developers in the form of JAR files.
There may also be a need to execute a command before of after a build occurs.
Sample commands to execute could include emailing status, setting up environment variables, deleting files or creating setup packages.
To handle this build and dependency systems were created. The more popular build and dependency systems are
Maven,
Gradle and
Ant.
I think that first came
Ant, then came Maven and finally Gradle. Each build system has their own idiosyncrasies, faults and strengths.
For more information regarding build systems and tools you may want to check out the build system's home page.
Some people who know these tools hang our around on the CodeRanch build areas found here
https://coderanch.com/c/build-tools.
You should really become more then just aware of one or more of these systems. I believe that this BlackJack may be using Maven and/or Gradle.