A master resumé is a place to collect all the information that you might put on a resumé or talk about in an interview. It helps you keep track of details about your experience, education, and other activities so you don't forget anything.
Your master resumé is a living document. If you keep adding to it throughout your career, you will always have a complete and up-to-date record of everything you have done. Having these details at your fingertips will make your future work search activities easier.