A cover letter is a document that you send with your resume to provide demonstrate your skills and experience for the job you are applying for.
A resume contains your work history and skills. It is usually same for all the jobs you apply for.
Whereas a cover letter is written specifically for the job you are applying for.
An effective cover letter is used to:
Do not copy-paste same cover letter for every job application. Make sure to customize it for each job you apply for.