What does 'big deal' mean?

This podcast episode, "The English We Speak," features Feifei and Rob discussing the English idiom "big deal." Rob initially uses the phrase sarcastically to express his excitement over a small discount on biscuits, prompting Feifei to explain that "big deal" is used to convey a lack of impressiveness. They provide examples of its sarcastic use and also introduce the phrase "no big deal" to signify something not serious. The conversation further explores "making a big deal about something" to mean making a fuss. The episode concludes with Rob revealing he bought 100 packets of chocolate digestives, which finally impresses Feifei.