It just means you did more than your body is used to. Like Foo mentioned, there is a current theory as to what causes it, but strangely, no one seems to know for sure exactly why it occurs.

It's not necessarily good or bad, however. It just means you did more than what your body is used to/was expecting. It can be bad if it interferes with your activities the next day (like if you have a BJJ class or something), but it's not really bad other than the fact that it can hurt and it can be annoying.

But it shouldn't be the goal. You can have effective workouts and never experiences DOMS.

And all else being equal, DOMS seems to decrease over time. After your first workout ever or after a long time off, you usually get really sore the next day or so. But after your body gets used to it (a few workouts), you only seem to get minor, if any, DOMS, even despite using heavier weights.