The word ‘really’ is primarily an adverb. It is used to modify verbs, adjectives, or other adverbs, often to intensify the meaning of the word it modifies.
For example, in the sentence, ‘She runs really fast,’ ‘really’ modifies the adverb ‘fast,’ indicating the extent of her speed. Similarly, in the phrase ‘That was a really good movie,’ ‘really’ modifies the adjective ‘good,’ suggesting a high degree of quality.
In addition to its adverbial usage, ‘really’ can also function as an interjection in informal contexts, expressing surprise or disbelief, such as in a conversation: ‘Really? I can’t believe it!’ However, its primary grammatical role remains as an adverb.