Non-commercial refers to licences that prohibit commercial use, according to some definition which is usually particular to the licence in question.
Non-commercial usage is variously defined as usage that is not primarily for-profit, usage in organisations that are defined as non-profit-making according to local taxation practice, and usage that doesn't involve taking money. Other definitions exist, and there is no general agreement about which is applicable, and whether the whole idea is desirable.
The most common ones are Creative Commons' BY-NC-SA and BY-NC-ND. We have a meta question about whether these and related licences are on-topic for this site, and at the time of writing the accepted answer is that they are often not. That is not to say that every question about non-commercial licenses is off-topic, but if all you want to know is how to licence your code in such a way that nobody can make any money off it, this may not be the place to ask.
