The computing industry often recycles terms. Context is everything.

The two kinds of “module” you present are unrelated, orthogonal issues.

• The Java Platform Module System (JPMS) is a way to identify to the Java compiler namespaces amongst all the classes and methods available at runtime.
• Multi-module in Apache Maven is a way to conjoin into one project what could be handled as separate projects. Each module in the project has its own POM with its own dependency and build settings, yet all can be managed as one super-project when combined as a Maven multi-module.

Very simple apps in Java may use neither.

• Ideally new Java apps would use the JPMS, but it is still technically optional. In a perfect world, JPMS would have been included in the original Java, but was in fact added only recently, in Java 9. JPMS is quite handy if your app will run as a standalone, with a JVM bundled, because you can include a JVM that has been stripped down to only the parts actually used by your particular app (see jlink and related tools enabled by JPMS).
• Maven multi-module projects are generally only used for complicated projects such as an app that includes a piece of functionality which may be spun-off for use in other projects. Or a multi-module Maven project may be good for an app that combines both a frontend user-interface module along with a backend business-logic module where we want to manage each part separately yet combine them into a single deliverable, such as a Vaadin Flow web app.

I can see how you could become confused, as both have to do with arranging classes. To oversimplify, Maven modules are about compile-time (dependency management and build automation) while Java Platform Module System is about runtime.