"Really" is a word used to express a strong or absolute agreement, or to emphasize the truth or accuracy of a statement.
Etymology: The word "really" has its origins in the Old Engpsh word "rǣdlīce" which means "truly" or "indeed".
Examples:
She is really smart. (她真的很聪明)
I really need a break. (我真的需要休息)
It's really hot today. (今天真的很热)