Really

Really is an adverb. It adds emphasis to the meaning of a sentence.
Ex:
(1) "I'm really sorry that I forgot to return your book." (really = "very")
(2) "There's really no need to apologise. I didn't need it." (really = "absolutely")

Often really goes in front of the auxiliary or BE. This is not the usual middle position for adverbs.
Ex:
(1) I really can't believe he's serious.
(2) You really should be more careful.

At the front of a sentence, really often indicates that the speaker is shocked, or disapproves of something.
Ex: Really, I'm terribly disappointed by your behaviour.

In a reply to a statement, really expresses surprise or polite interest.
Ex:
(1) Is that really true?
(2) "John is giving up his job and becoming a priest." "Really? I didn't know he was a religious person."

Note: Really is an adverb of degree making the meaning of an adjective or adverb stronger.
Ex: It was a really exciting race. --- She cares really deeply about her work.

I hope this article helps. If you have any question, please leave a comment below.