I first heard about "pH balance" when I was a lifeguard at a pool during college — I had no idea pH balance also applied to my own body! That information came to me years later.
I first heard about pH balance and the body when I was reading Thrive back in 2008. My eyebrows shot up almost immediately and within a few pages, I put the book down, never quite completing that chapter. It was just too daunting. I was confused and overwhelmed and figured, I'll just keep eating plants and hope for the best (I like the simple approach!).
Then two years later (no exaggeration), a friend of mine who'd recently adopted a raw diet mentioned in passing that if you're alkaline, a shot of wheatgrass will taste sweet and if you're acidic it'll taste... bad. This got my gears going. I wondered if this could explain why I thought kale was so delicious and my omnivore friends thought it was bitterand disgusting (I'd read far enough in the Thrive chapter to know most animal products are acidic and most plant foods are alkaline).
That got me curious, and I started reading more about pH balance. Finally, about a year ago, I came across this great pH balance video that made it so easy to understand. To put it simply, the body has a pH balance. Everything we eat or drink affects this balance. Some foods make the body more acidic while others make it more alkaline, and what a food does might surprise you! For example, oranges are alkaline (not acidic) when consumed. See below: 
Meanwhile, our bodies must maintain a slightly alkaline pH, so if you're eating a whole foods, plant-based diet, this shouldn't be a problem. You'll notice in the chart above that most plant foods are alkaline (with some exceptions). However, if you're eating junk foods (even vegan junk foods) or animal proteins, you're pulling your balance in the acidic direction.
Why does that matter?The body is fierce about protecting and stabilizing its slightly alkaline pH balance, so if you're eating foods that pull the balance in the wrong direction, your body will start pulling minerals (alkaline) from your bones to counteract the acidic onslaught. This can lead to osteoporosis (source).
Everything I've read about pH balance and diet seems to suggest your diet should be 70-80% alkaline foods. I've never done the math on my own diet (but I have tested my pH balance before), and there are some criticisms about the science behind pH balance and if it matters. So instead of getting invested or overwhelmed again, I look at the pH balance of foods as another helpful tool for making great choices.
I mean look at kale! He's a solid 10!
Here's a chart of how foods rate on a pH scale for those looking for a more detailed listing of each food.
