Reddit reviews Agenda 21

Antivaxxers believe Agenda 21 is a conspiracy, but in reality it's a non-binding action plan of the United Nations with regard to sustainable development, mostly relating to developing nations. It's a big plan, covering things like fighting poverty, conserving the environment, protecting the vulnerable, and implementing all those things with science and technology. Not a thing about depopulating any first-world countries, or really much about first-world countries at all.

Coincidentally, it's also a novel in the "thriller" category by Glenn Beck (actually written by Harriet Parke). So I guess the first thing to ask a conspiracy theorist if they're talking about the sustainability plans, or the work of fiction Glenn Beck didn't actually write.

There is a neat website about the original Agenda 21, if you're interested in that sort of thing.