What does the | (pipe) operator do?

Question

My colleague forwarded me some code as below. I want to understand how symbol | operates. I have a rough idea that it is looping through NutritionData Tuple and storing unique values of food into string Foods. But is there something that I am missing? I searched for | online but wasnt able to find anything.

tuple FoodNutrition
{
    key string food;
    key string nutrient;
    float amountPerServing;
};


{FoodNutrition} NutritionData = ...;

{string} Foods = {nd.food | nd in NutritionData};
{string} Nutrients = {nd.nutrient | nd in NutritionData};