That’s an old principle, often attributed to David Hume if I’m not mistaken. It means that there’s no chain of reasoning that takes you from factual statements about the way the world is to normative statements about the way things should be.
That’s not to say that factual statements are irrelevant to ethical questions, just that when you’re engaged in ethical reasoning you need some sort of additional inputs.
Religious traditions often give such inputs. For the non-religious, one common point of view is utilitarianism, which is the idea that you ought to do the things that will maximize some sort of total worldwide “utility” (or “happiness” or “well-being” or something like that). The point is that in either case you have to take some sort of normative (“ought”) statement as an axiom, which can’t be derived from observations about the way the world is.
For what it’s worth, I think that Hume is right about this.
The reason I’m mentioning all this utterly unoriginal stuff right now is because I want to link to a piece that Sean Carroll wrote on all this. In my opinion, he gets it exactly right.
Sean’s motivation for writing this is that some people claim from time to time that ethics can be (either now or in the future) reduced to science — i.e., that we’ll be able to answer questions about what ought to be done purely by gathering empirical data. Sam Harris is probably the most prominent proponent of this point of view these days. If Hume (and Sean and I) are right, then this can’t be done without additional assumptions, and we need to think carefully about what those assumptions are and whether they’re right.
I haven’t read Harris’s book, but I’ve read some shorter pieces he’s written on the subject. As far as I can tell (and I could be wrong about this), Harris seems to take some sort of utilitarianism for granted — that is, he takes it as self-evident that (a) there is some sort of global utility that (at least in principle) can be measured, and that (b) maximizing it is what one ought to do.
Personally, I don’t think either of those statements is obvious. At the very least, they need to be stated explicitly and supported by some sort of argument.