A cover letter is a short (usually one page, 250-400 words) letter that accompanies your resume. It's not a restatement of the resume — it's a targeted explanation of why you want this specific role at this specific company, and which two or three experiences make you the obvious fit.
Structure: a concise opening (one or two sentences naming the role and your motivation), a middle that maps two or three of your achievements to the role's requirements, and a close that states interest and next steps. Address it to a named hiring manager if you can find one; skip generic "To whom it may concern" openings.
Cover letters are still expected for most roles above entry level — especially in traditional industries (finance, consulting, government, nonprofit) and at smaller companies where a named hiring manager will actually read it. AI tools like HatchCV's free cover letter generator can draft a strong starting point from your resume and a pasted job description in seconds.