This is the 12944th most frequent Russian word.
лимонную
"Lemon" (adjective form, related to lemon).
Here, 'лимонную' is used as an adjective describing the type of pastry, specifically a lemon-flavored one.
Я купила лимонную пирожек.
I bought a lemon pastry.
In this sentence, 'лимонную' describes 'кислоту', indicating the type of acid, which is derived from lemons.
Он добавил лимонную кислоту в рецепт.
He added citric acid to the recipe.
The word 'лимонную' is used to modify 'воду', presenting the drink as containing lemon as an ingredient or flavor.