There's a scale which from 1 to 14 which shows how acid and how alkaline various things are.
For example, the blood in the human body rates at about 7.2 on the pH scale - so essentially it's very balanced. Neither acid nor alkaline.
Foods have been rated on this scale, too. It's important to know the acidity and alkalinity of foods you're ingesting, so that you keep your body in perfect balance. Stomach aqueous fluids are quite acidic with hydrochloric acid, giving it great digestive properties. Yet the remainder of the body needs to be balanced, or in a slightly alkaline state, and retain that homeostasis.
However, when blood pH becomes acidic, a person can die almost immediately. Cancers also grow well in an acidic state, as cells are broken apart, chemically. Viruses can replicate quickly and proliferate in life-threatening cancers.
Rather than getting into all the chemistry of how this happens and where the variations occur, all you really need to know are two things: some minerals are alkaline while some are acidic, and you can easily control your acid/alkaline homeostasis through proper diet.
There are several pH scales online and in print ... but basically it's easier to remember that sugars and foods that break down into sugars quickly (like white bread, white milk, processed crackers and pastas, etc.), as well as artificial sweeteners, are highly acidic! Red meats and pork are quite acidic, too. Next down the list are chicken and fish. Then getting closer to 7 are whole grains like brown rice, and quinoa and millet, with fruits being slightly acidic. Then on the alkaline side (THE HEALTHY SIDE) are vegetables - in their raw state mainly.
So -- the best diet is eating raw vegetables with a few fresh fruits, and a little bit of whole grains.
Second best thing -- supplement with calcium, magnesium, and potassium. If you're eating red meat, or a protein bar, be sure to take some high quality calcium, magnesium pills - powdered form in a gel cap.
Stay healthy and happy!