Neutral means neither good nor bad. For example, someone invites a friend along to liberate animals, but the friend does not understand the merit and the benefit related to this activity, just goes along having no particular purpose. After the activity has ended, the friend’s participation would have resulted in a kind of neutral karma. Maybe someone will question, “Didn’t the friend also save some lives? Why was this karma neutral?” It is because the friend did exactly the same thing as everybody else but with no idea as to why it was done. That makes it neutral.

- Quote from The Right View, "The Three Supreme Methods"