I fell off a horse the other day. It bruised my confidence; it hurt my backside; and, because it happened while my horse was going over a jump, I didn't exactly want to go over that jump again, not right after. But I did. I hauled myself up into the saddle, rerode the course -- and jumped it.
And now? If I ever have to write a scene where my protagonist falls off a horse, I'll know how to do it. (I'll probably need to at some point, actually. My current WIP involves horseback riding and mystery and superstition and family.) I'll know the emotions that go hand-in-hand with the experience. I'll know which muscles ache the most an hour afterward, and five hours afterward. And I'll know how hard it is to get back into the saddle again.
This, I believe, is at the core of the phrase "write what you know": use your own personal experiences to create scenes that ring emotionally true for whoever reads it.
It's not just about having the physical knowledge. After all, I don't have to have physically gone to Starbucks and ordered an iced cappucino to know how to write a scene involving one such trip. It's more to do with how your character will feel throughout the experience. Maybe a local Starbucks has a particular atmosphere -- shades hanging low, lamps lit, French vanilla scent especially strong -- that reminds your character of home. And these emotions can be worked in at the best of moments to connect the reader with the character.
This doesn't always apply, of course; sometimes, a scene isn't consequential enough to warrant a living, breathing description. Or how about fantasy and the supernatural: who can say they've ever experienced being a vampire? In these cases, making up the experience is necessary. What you won't want to make up are the emotions that go with the experience.
So jump a horse. Learn to climb trees (again). Set up a blind date. And remember that both failures and successes are experiences that everyone goes through, and good writers use all experiences to their advantage.