Acidulated water is water with added lemon juice. When preparing some fruit (apples and pears) and vegetables (artichokes and potatoes) they oxidise - turn brown - when cut or peeled and left exposed to the air for just a few minutes. Popping them into acidulated water after peeling prevents the discolouration.
Don’t leave the fruit or vegetable sitting in the water for more than a few minutes otherwise it will absorb too much water.