In computing, to port means to transfer (software) from one system or machine to another.
"the software can be ported to practically any platform"
So when saying "Microsoft ports open source Java to Windows 10 on ARM" means that Java will take charge of the Windows 10 ARM platform.
Microsoft’s Java Engineering Group initial changes are being upstreamed to the OpenJDK project itself.
An early access binary is available via GitHub:
Developers can start building with Java on Windows 10 ARM64-compatible laptops,
with core Java extensions in Visual Studio Code for ARM64, along with tools such as Apache Maven and Gradle.