There's this sentence : "some people think Chicago's nickname is from politicians who talk too much
I need to know what is the meaning of politicians talk too much
And why politicians?
This is a joke.
"Politicians" are people who have an elected office in a legislature or senate (for example). Their job involves lots of discussion. So they have to talk a lot. Some people think that politicians should talk less and be more practical. They say that politicians talk too much, and that much of what they say should not be trusted. A colloquial paraphrase would be that they are full of hot air.
"...who talk too much" is a relative clause that describes "politicians".
A slang term for a person who talks a lot is "windy". Chicago is famous for its wind, and so its nickname is "The Windy City".
The joke is that the politicians in Chicago are "windy".
As @James K explains, it is a play on Chicago’s nickname, The Windy City, based on its particular weather, and on the metaphor of wind for (excessive) talk.
Some other examples of this metaphor, which is common in English, are
Another metaphor with similar meaning is of a mechanical device. It gives us such examples as