Cover letter
In short
A cover letter is a short message accompanying your resume that explains why you want the role and why you are a strong fit, in your own voice.
A good cover letter is three to four short paragraphs: a hook, evidence that you can do the job (with a specific example), why this company, and a close. Tailor it to each posting — a generic letter is worse than none. For remote roles, it is also a sample of your written communication, which matters a lot on distributed teams.