Java is software first released by Sun Microsystems in 1995. It is the underlying technology that powers state-of-the-art software including games, utilities, and business applications. It is avaliable for free for many different computer systems.
Rummi will only work if you have Java installed on your computer. Because Java is so widely used, it is likely that Java is already available on your pc.
But if Rummi doesn't work, or if you see a message that there is no Java on your computer, then you have to download and install it first.
For Windows computers, you can download Java from Oracle's Download Java page.
For Macintosh (OS X), Apple supplies their own version of Java. Use the "Software Update" feature (available on the Apple menu) to check that you have the most up-to-date version of Java for your Mac.
For Linux, you have several options. One is to download Oracle's Java for Linux.
You could also choose for the OpenJDK/IcedTea Java implementation. You'll probably find the option to download and install this package in the software update manager of your computer.
On other systems, including tablet computers or other handheld devices, Java may be available too. Please consult the support pages for your system, or search the internet for a suitable Java package. If Java is available for your computer or device, then it is likely that Rummi runs on it, too!