What acid means
In cooking, acid means any ingredient that adds sourness. Common examples are lemon and lime juice, vinegar, wine, tomatoes, yoghurt, buttermilk, soured cream, tamarind and many fruits. Acid is one of the basic building blocks of flavour, alongside salt, sweetness, bitterness and savouriness, and cooks rely on it to make food taste lively. A squeeze of lemon or a dash of vinegar can lift a dish that tastes dull or heavy, cutting through fat and balancing sweetness.
That is why recipes so often end with an instruction to adjust the seasoning with lemon juice or vinegar. Acid also changes food physically. It firms up proteins, which is how lime juice turns raw fish opaque in ceviche, and it helps break down tough fibres in marinades. In baking, acidic ingredients such as buttermilk or yoghurt react with bicarbonate of soda, called baking soda in the US, to produce bubbles that help cakes and soda bread rise.
Acid slows the browning of cut fruit and vegetables, but it dulls the colour of green vegetables if added while they cook, so timing matters. You will see acid mentioned in recipes, cooking guides and discussions of flavour balance.