According to All Acronyms, RDA is most commonly used as an abbreviation of the term "recommended daily allowance" or "recommended dietary allowance." The recommended dietary allowances are established by the Food and Nutrition Board of the National Academy of Sciences.
The recommended dietary allowances are designed to provide the proper nutrition to keep those who follow the guidelines healthy. The first recommendations were published in the 1960s, but they have been periodically updated and adjusted, according to Wikipedia. The best way to obtain enough vitamins and minerals is to consume enough fruits, vegetables, grains and protein sources, rather than to take supplements.