'really' vis a' vis 'very' is the very colloquial way of saying just that, 'very', but also with added emphasis. So, some 14 year old girl talking to her girlfriend might be telling her about this new boy at school who is 'really dreamy'
For someone older to say, "He's very good looking" is rather a statement of fact. To say, "He's really good looking", then the listener understands he is very very good looking!
Never use 'really' for 'very' in formal conversation or writing. (Leave exceptions to this until you're acculturated and now the right moment!.)
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