It is a way to connect two sub-clauses, enabling you to talk about about the first sub-clause in the second one.
from Quotation+言う It derives its meaning from 言う in that it talks about (と) "the thing called (...)".
人間という生き物はバカだな The beings that are referred to as 'humans' are stupid.
これは日本語でなんと言う言葉ですか？ what is this word in Japanese? (lit. This word is what word in Japanese?)
It is often combined with either The "..." one, Single instance of performing an action or the act of doing (...)
==日本人はお酒に弱いと言うの==は本当？ Japanese people being weak to alcohol, is it true?
独身だと言うのは、嘘だったの？ You being single, is it a lie?
リブートと言うのはパソコンを消えて、点けると言うことです。 The thing that is called 'reboot' is the act of "turning it off, and turning it on".
It is said that (...)
When used with the 〜ている, or passive (言われている) it implies it is currently still being said. implies that a lot of people (in general) say so- it is a generally understood thing; "It is said that"
日本語は難しい言語と言っています。 It is generally said that Japanese is a hard language.
ナットは体にいいと言われています Natto is generally regarded as a food that is good for your health.