I often have seen using "meaning of something". So I understood I have to use "meaning of", but I saw following sentence:
Its meaning is "Really?"
I think why using "its meaning"? What is the difference between "its meaning" and "meaning of it"?
"Its" is the possessive form of the personal pronoun "it". You can say "meaning of something", but it's unusual or rare to use the preposition "of" in front of a personal pronoun. Instead, you use the possessive form. For example:
Instead of saying "I don't know the meaning of it", you say "I don't know its meaning". The use of "its" is far more common and idiomatic.
Its meaning is "really".
In this sentence, "its meaning" is the subject.
The sentence is grammatical, but "It means really" is more common.