1 Martian 2: I’m concerned about this plan. Have you heard of the Prime Directive?
2 Martian 1: No, what’s that?
3 Martian 2: It’s a principle that says we shouldn’t interfere with beings of a more primitive culture.
4 Martain 1: You mean humans?
4 Martian 2: Well, not all of them, but definitely college students.

The Prime Directive comes to us from Star Trek. It's a standing order for Starfleet personnel that prohibits them from interfering in the natural development of alien cultures and civilisations. The exact wording of the Directive is never given in any canon source, but the general principle has been paraphrased many times. It generally applies only to civilisations that have not yet developed the technology to determine for themselves that other alien civilisations exist. So the idea is to prevent starfarers from contacting less technologically developed civilisations and giving them either advanced technology, or the knowledge that other civilisations exist. The justification for this prohibition is that such contact would irreparably contaminate the natural development of such civilisations. (Which would be a bad thing.)

Interestingly, I had occasion to discuss the Prime Directive in my online ethics classes for children. We recently had a lesson on contacting alien life, and I asked them to consider the Prime Directive, and decide whether they thought it was a good idea or not. (Earlier questions in the same class were about finding bacterial life on Mars, and about finding multicellular but non-intelligent life on a hypothetical planet humans had selected for colonisation.)

The responses were interesting. Of the 40 or so kids I teach, about half of them thought the Prime Directive was a very good idea. But the other half thought it wasn't good, because they said it would be a good thing to contact intelligent civilisations and bring them knowledge and advanced technology. Some mentioned that it could help the contacted civilisation avoid wars, and pollution, and climate change, and other problems. And one of them said that if some advanced alien civilisation was out there watching us, but deciding not to contact us because of some Prime Directive, that would be terrible, because we really need the help.