Quite simply, a "hook" captures the reader's attention.  A hook might consist of a compelling anecdote, a bit of dramatic dialogue, an example, an analogy, a quotation, etc.  Besides providing a more engaging start to your essay or paper, a hook can function as an organizing principle for the entire work or as an idea that you may refer to again in the conclusion.  For example, one student began a research paper by presenting the story of a friend who had been seriously injured in an accident and was now undergoing occupational therapy.  At the end of the research paper (on occupational therapy) she finished the story of her friend who--thanks to the therapy she received from an occupational therapist--was able to go back to work and lead a normal life.  By opening her research paper with this story, she immediately drew the reader in, provided a dramatic reason for her interest in occupational therapy, and created an organizing principle for her paper, e.g., the story set the stage for demonstrating the importance of occupational therapy and then developing the types of procedures it uses to rehabilitate patients. So, before you start your next essay with a deadly sentence such as "This paper treats the effects of pollution on plant life" consider how you might create a hook that would open your paper more engagingly. .