When you compile C or C++ code, you usually compile it directly to native machine code and link it with libraries that are specific to the operating system. So if you compile your C program on Windows, you will get an *.exe file with Intel x86 machine instructions in it, and containing calls to Windows operating system functions. You cannot run that program on a different operating system or on a system which doesn't have an Intel x86 CPU.
Contrast that to
Java: you compile Java to bytecode, which runs on the JVM. The bytecode can run on any computer or other device where there is a JVM, it doesn't matter what kind of CPU the device has. Also, all the classes and interfaces in the standard Java library are guaranteed to be available.
Note that in principle you could compile C or C++ to bytecode too. There's nothing inherent in the programming languages that makes them necessarily platform-dependent. It's just how C and C++ are commonly used. C and C++ have a number of features that make them efficient languages, but also harder to make programs that compile on different machines and operating systems. For example, the size of an
int in C and C++ depends on your compiler and target CPU and operating system. On some systems, an
int is 32 bits, on other systems it's 64 bits and on other systems it might be only 16 bits. (In Java, an
int is always 32 bits, no matter what CPU the program is going to be running on).
