an adverb is a word that modifies or describes a verb, an adjective, or another adverb. In the case of ‘rapidly’, it modifies the verb ‘change’ to indicate the speed with which the change occurred. Adverbs that end in 'ly' often indicate manner or degree.
1. Technology is changing rapidly.
2. The wind blew rapidly across the lake.
3. She read the article rapidly, in an effort to finish it quickly.