A portable C program:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    printf("Hello, World!");

    return (EXIT_SUCCESS);
}

You can take that C program and compile it on any machine with a C compiler and have it work (assuming it supports printf... I am guessing some things out there may not).

If you compile it on Windows and try to run that binary on a Mac it won't work.

The same sort of program written in Java will also compile on any machine with a Java compiler installed, but the resulting .class file will also run on any machine with a Java VM. That is the architectural neutral part.

So, portable is a source code idea, while architectural neutral is an executable idea.