What does this idiom mean: "a lot of hot air"?

[Student]

Test No. incompl/inter-18 "Hot and Cold", question 4

They like to think they're very important people but politicians just talk a lot of hot ......... most of the time.

(a) words
(b) steam
(c) air
(d) stuff

Test No. incompl/inter-18 "Hot and Cold", answer 4

They like to think they're very important people but politicians just talk a lot of hot air most of the time.

Correct answer: (c) air

Hello everybody! I didn't understand the exc.
'just talk a lot of hot air most of the time"
help me please, friends
thanks
Student

[Alan]

Hi Student,

'A lot of hot air' means a lot of talking without really saying anything very important. The sort of thing that politicians all over the world are good at!

Alan
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English as a Second Language
You can read my ESL story Conditionals

[Student]

Hello Alan.
Thank you very much
Student

[Clara]

hot air = empty talk
haha

[Kisaki]

Hi
I still have a wonder. What is the meaning of the word "but" in here?
Thanks
Kisaki