Adverbials are words or phrases that provide additional information about a verb, adjective, or another adverb in a sentence. They answer questions such as how, when, where, and why an action occurs. For example, in the sentence "She runs quickly," the word "quickly" is an adverbial that describes how she runs. Adverbials can be single words, like quickly or silently, or phrases, such as in the morning or with great enthusiasm. They help to create a clearer picture of the action and can be placed at different points in a sentence, allowing for flexibility in writing and speech.