Meaning: This sentence expresses that the thing being referred to has the quality of being large in size or quantity.
Choice of Words: The word "it" is used to refer to something that has already been mentioned or is known to both the speaker and the listener. The word "is" is a form of the verb "to be", which is used to describe a state of existence or condition. The word "big" is an adjective that describes something that is large in size or quantity.