RDA is a food-and-health figure: how much of a nutrient — a vitamin, a mineral, protein, and so on — a typical person is advised to take in over one day. You see it on the side of a vitamin bottle, in a school health book, or on a diet chart. The letters come from Recommended Dietary Allowance, a US scientific name. Though actually, teachers, nurses, and packet designers very often say Recommended Daily Allowance instead, because daily is the word people already have. Both expansions point to the same kind of number; dietary is the older official one. An RDA is a guide, not a promise and not a law. It was built for a normal healthy adult, so a child, a pregnant woman, or someone who is ill may need more or less. Eating a bit over or under on one day is normal; the idea is the pattern over time. You will also meet newer names for the same job, such as DV on some US packets and NRV on many Taiwan and European labels. Other groups use the same three letters for completely different things, so if you are not in a food or health context, check.