J2ME is a subset of the Java specification, specifically adapted to embedded devices like Mobile Phones. It is the platform on which all of the mobile games from the early 2000s run. As it uses a Java Virtual Machine, programmes written for J2ME are device and Operating System independent to a certain degree and can therefore be easily ported to a wide array of different devices without much (if any) adaptation.
With the advent of newer and more powerful Smartphones since 2010 and onward, the J2ME standard became somewhat obsolete. Newer games run in native ARM or X86 code without the need for a Virtual Machine, which increases speed but also makes theses apps more difficult to port to other systems.