The phrase "big deal" is an idiomatic expression that means something is important, significant or worth noting. It can be used in a positive or negative way to convey one's opinion about a situation or event. For example, if someone says "this is a big deal," it implies that the matter at hand is of great importance and should be taken seriously. On the other hand, if someone says "it's no big deal" it means they are downplaying the significance of something and suggesting that it isn't worth getting too worked up about.