An attention grabber is a word or phrase that is designed to capture an audience's attention. An attention grabber is often used at the start of a persuasive essay and is called a "hook."
Anything can be an attention grabber depending on the context. Any phrase or sentence that sparks curiosity in the reader will cause the reader to intently focus on the following sentences and paragraphs. An important part of an effective hook is to contain relevance to the following content. If the hook is relevant, readers will keep reading in order to find an answer, but if the hook is obviously only included to peak curiosity, focus will rapidly trail off.