A resume is the representation of your professional self. It is a concise compilation of your educational and professional experience, as well as the skills that make you desirable for the workforce. It is what you use to sell your merit and skills to potential employers.
Your resume is arguable the single most important part of the application process. A well-organized, relevant resume will set you up to get an interview, while a poor resume will get completely lost in the sea of applicants. This is a fairly modern concern, as job postings now get thousands of online applications a day.