A great deal of what you write is intended to convince the reader that you have an important point to make. When you write a letter applying for a job you want to convince the reader that you are the right person for the job. When you write a review of a film you want to convince the reader that you have something important to say about the film, and maybe you recommend it, or, on the other hand, suggest that it is not worth seeing. In an essay on some aspect of American government you want to convince the reader that you can answer the questions that have been posed and that you can throw light on specific aspects of American government. In all these three examples you want to show your reader that you have something sensible and important to say about the topic that is under discussion.You do this by arguing your case. You offer “a line of argument” keeping it within the framework of the chosen topic. For example, your letter of application for a job has a presentation of yourself and your qualifications as its framework. “You” are the topic! It might therefore be relevant to mention your hobbies in your letter. If you are applying for a job in a bookshop it would be sensible to point out that reading is one of your hobbies, if it is. You include this in the line of argument running through the letter, perhaps giving this information after you have listed your education and other formal qualifications. You do not, however, spend a paragraph writing about your brother’s or sister’s hobbies. That would be irrelevant. Information about them falls outside the framework you have constructed.
Similarly, if the topic is American government and the question is “Does the President have too much power?” you do not write about American geography or American sports. You write about the mechanics of political power in the USA, showing step by step whether, in your view, the President does or does not have too much power.
You must, then, avoid irrelevance. Keep a sharp focus on your topic.
