Here's why:
* It modifies verbs, adjectives, or other adverbs: It describes how something is done (e.g., "He improved significantly" - modifies the verb "improved"), to what extent (e.g., "The price is significantly higher" - modifies the adjective "higher"), or how much (e.g., "He ran significantly faster" - modifies the adverb "faster").
Let me know if you'd like to see more examples!