Basically, the idea is that the body prefers a slightly alkaline pH (stay with me now). Rice, meat, cooked foods, coffee, black tea and smoking all acidify the blood, where as fresh vegetables and green tea de-acidify it. Acidified blood then leads to cancer, heart disease, and other chronic ills.
Ok, but the body naturally maintains blood pH in a narrow range. If this theory has any truth to it, then it wouldn't be "acidified blood" that's bad for you, but the energy and nutrients consumed, and the byproducts created and disposed of, in the course of maintaining proper pH against an acidifying diet. Low-pH urine might be a sign of the body working overtime to keep the blood neutral.
To this end, you can buy water ionizers that produce alkaline water, and drink that rather than normal water. According to the product literature I saw (my neighbors in China had one installed--a lot of money for a retired couple), the original research looked at villages around the world where people lived longer than their average countrymen. The common thread was slightly alkaline water (supposedly). I went online to check sources, and indeed, there was a lot of university-affiliated research along these lines in Japan.
This was 6 years ago. I'm sure you can find more information online, if you are curious.