To acidify something is to chemically turn it into an acid or make it more acidic. Farmers often acidify soil by adding fertilizer to it.
When you acidify something, you increase its acid content or make it more sour. Too much carbon dioxide dissolving into oceans acidifies them, one of the many negative effects of global warming. When it comes to food, you're most likely to acidify a dish by adding citrus fruits, like lemons, which taste sour. The root word of the verb acidify is the Latin acidus, which means "sour or sharp."