Well, for starters, passion is just a stronger version of interest. What is it that I care about? Do I care enough about it to spend significant time on it? Would I spend money to do it? If the answer is yes, you're circling around your passion.
Let's say for instance, that you've always been interested in community development. You believe strongly that communities should be built in ways that allow its members to thrive and live well, but you're painfully aware that not all of them are. You've spent your summers interning at housing development firms, traveling to understand the world in a more robust way, and seeking to expand your knowledge base so that you can positively contribute to strong communities (in addition to sleeping in and getting fully caught up on your favorite reality shows, to say the least).
So what should you write about? How can you display that your passion today will translate into success tomorrow? Write about whatever it is you spend your time doing - this is your passion. No, not the reality show part or the 10 straight hours of sleep part, the part that you gravitate towards once you're all caught up on Real Housewives of <fill in the blank>.
Tell us why community development is so meaningful to you. How have you contributed to this field already? What kinds of leadership have you exhibited as it relates to housing and community development? And, of utmost importance, how is the school that you're applying to uniquely matched with your passion? This one requires research. Are there Professors at your school of interest who teach and research in the same field that you're interested in? Does your school of interest have a particular Department of study that piques your interest? Hone in on this because this will show that you not only have passion, have already begun to exercise it, but have also done your homework to know that you're a good fit for their school. You see, admissions officers don't want to guess about whether you'd fit in; they want you to make it 100% apparent. They want to hear that you will be successful and it's in their best interest to admit you so that their name can be associated to all of the forthcoming positive changes that you plan to make in this world.
So go on, tell us what you're passion about. Be authentic, vulnerable, humble, and the sky is the limit.