It means to write with and about something you feel a great passion for. That passion will translate on the page into a story of great depth. It may be a passion for action films so you will write inventive and exciting sequences with compelling and original characters. It may be a story about a woman dying of cancer which will translate into a story of unique and original characters with honest and genuine emotions and struggles that pull us into her story.
Writing is hard at the best of times.
There is tremendous rejection and endless hours alone in the glow of your computer so if you aren't writing about the things that you feel passion for then you are cheating yourself and your readers of exploring and enjoying a great story.