Before Java 9 and Project Jigsaw, you would have to package the entire monolithic Java Runtime Environment (JRE) to distribute your application with an embedded JRE runtime. This took up a lot of space. Now, you can generate your own custom runtime with only the modules you want to keep the size of the final package smaller.
You can think of a runtime image as another name for a Java runtime environment (JRE). When you create a custom runtime, you are generating a new JRE that only includes the modules you want. You can exclude core java modules that you don't need and you can include custom modules that you wrote. They will all be "baked in" to the runtime.
If you are familiar with Python Virtual Environments, it is similar to that. It is a directory that contains an isolated Java environment with only the specific dependencies needed that will not conflict with other isolated runtimes.
After following this guide, you will understand how to use
jlink to create runtime images with only the modules you want. You should also know how to add custom modules including third-party modules like JavaFX.