For some reason I think I'd be embarrassed to put in translations for this, but for the brave one, it's Reimu: That tickles~ / Marisa: I'm getting my daily salt allowance from your armpits, Reimu (+da ze)
Twelve years later, because time is meaningless on the Internet: It's nutritional jargon, as in the phrase "recommended dietary allowance", and refers to the amount of a given nutrient that the U.S. National Academy of Sciences sees as necessary for proper nutrition in an average adult. The RDA system was developed during World War II as part of wartime food rationing, to make certain that the government rationers wouldn't inadvertently cause mass malnutrition. Nowadays they've largely been replaced on food labels by "Daily Values", which are based on a more modern set of guidelines, but at the time this comic was drawn and translated, the RDA system would still have been in common usage.
Strictly speaking, there isn't an RDA for "salt" as such; what Marisa's really referring to is her daily allowance of sodium, the chemical element in salt that is considered a nutrient.
(It would be interesting to know whether the original Japanese text refers to it in those terms, or uses some Japanese government equivalent of the period. Then again, it's possible that the Japanese use the American RDA system, since it would have been the Latest Thing right after the war, when Japan was occupied--and in large part reinvented, governmentally speaking--by U.S. forces.)
Pant, pant...I'm getting my daily salt allowance from your armpits, Reimu.That tickles~