I read a blog post about sterility. but i'm still not clear - what is it?

Unable to have kids: Strictly speaking, sterility is the total inability to get pregnant for a woman or conceive a child for a man, for example if both a woman's tubes are completely blocked or a man has zero sperm (azoospermia) then a child cannot be conceived naturally. It is not the same as infertility where there may be a lower chance of conceiving, but still some chance.

Pretty rare: Sterility is the absolute inability to conceive. With all of the tools we now have sterility is very unusual. Impaired fertility is not uncommon, but we can successfully help most couples conceive.