an adverb is a word that modifies or describes a verb, adjective, or another adverb. Really is an adverb that is used to emphasize a statement, usually describing the degree to which it is true. It can often be used instead of 'very' for a stronger effect.
Really is almost exclusively used to emphasize a statement, often making it stronger, to express intensity. The use of really does not change the grammatical structure of a sentence, but it does make it stand out more.
1. I really like this restaurant.
2. She worked really hard to get this done.
3. The movie is really interesting.
Really should be placed right before the word it is modifying, as is does not change the grammatical structure of a sentence. Really should not be used to replace words like 'very' or 'truly' unless the statement needs to be further emphasized. Really should not be used in place of adverbs like 'actually' or 'clearly' which connote different meanings.