Our body chemistry is healthiest at a pH of about 7.3, so slightly alkaline. If it becomes too acidic we become ill (when my daughter was diagnosed with Diabetes 1 last year her body pH was at 6.8). Overacidification makes us prone to cancer (tumors feed on sugar), diabetes, heart disease and all inflammatory conditions (overacidification causes inflammation).
The mainstream Western diet is too high in acidifying and too low in alkalizing foods and it's easy to become imbalanced. So let food be thy medicine to stay healthy or heal yourself - low tech, low cost. Food comes in all colors of the pH spectrum - it is simply a matter of awareness in order to eat more of the beneficial ones. In broad strokes the foods that acidify the body chemistry are sugar (as a culture we are literally addicted to sugar and meat!), meat (the redder the more acidifying) and refined carbs (as they transform immediately into sugar). Ha - you'll say. Exactly.
The foods that alkalize our body chemistry are most fruits and vegetables (not potatoes, and not bananas as they are both highly starchy - starch also turns immediately into sugar). The most alkalizing vegetables are all the greens. Since we thrive with a slightly alkaline chemistry we need to not only balance acidifying and alkalizing foods, but eat slightly more alkalizing than acidifying foods - hence the recent push for a more plant based diet. Of course each person is unique and you need to test for yourself which foods work better for you and which don't. Some people are better with breads and cheeses, others not so much. Some people are ok with reasonable amounts of pastured meats, others less so. Your digestive system and constitution are unique, and also tied to your geocultural and genetic heritage.
The main message is to increase the amount of vegetables (you heard that one before), especially the greenies, cut out the sugar (no news, really), and drastically reduce your meat (go meatless every few days, switch to fish, do poultry instead of red or pork) and refined carb intake (less bread, pizza, pasta, waffles, bagels, and baked goods). Instead, increase the amounts of healthy fats, nuts and legumes in your diet to meet your long-term energy needs (sugar is a false energy booster without nutritional value).