What is meant by namespaced content and what advantages does it have?

Question

I was reading this blog by James Bennett regarding HTML vs XHTML . He writes :

I don’t have any need for namespaced content; I’m not displaying any complex mathematical notation here and don’t plan to, and I don’t use SVG for any images. So that’s one advantage of XHTML out the window. I also don’t have any need for XML tools; all the processing I need to do can be handled by HTML-parsing libraries like BeautifulSoup. That’s the other advantage gone.

What does he mean by namespaced content and what advantage does it provide us ?