what is the meaning of java is platform independent language? what is it significance?

Java solves the problem of platform-independence by using byte code. The Java compiler does not produce native executable code for a particular machine like a C compiler would. Instead it produces a special format called byte code.
This looks a lot like machine language, but unlike machine language Java byte code is exactly the same on every platform. Java programs that have been compiled into byte code still need an interpreter to execute them on any given platform. The interpreter reads the byte code and translates it into the native language of the host machine on the fly. The most common such interpreter is Sun's program java (with a little). Since the byte code is completely platform independent, only the interpreter and a few native libraries need to be ported to get Java to run on a new computer or operating system. The rest of the runtime environment including the compiler and most of the class libraries are written in Java.

One of the key reasons Java technology is useful in a networked environment is that Java makes it possible to create binary executables that will run unchanged on multiple platforms. This is important in a networked environment because networks usually interconnect many different kinds of computers and devices. In a typical enterprise environment, for example, a network might  connect Macintoshes in the art department, UNIX workstations in engineering, and PCs running Windows everywhere else. Although this arrangement enables various kinds of computers and devices within the company to share data, it requires a great deal of administration. Such a network presents a system administrator with the task of keeping different platform-specific editions of programs up to date on many different kinds of computers. Programs that can run without change on any networked computer, regardless of the computer's type, make the system administrator's job simpler, especially if those programs can actually be delivered across the network.