What does "pile up (piling up)" mean?
Here the term "pile up" is being used to mean arrange into piles or stacks. When we "pile" something it means we lay it on top of another thing, and "up" lets us know the direction this "pile" is going in. So by saying "piling it up" we know that people are stacking or placing their food on their plate, each item on top of the other to fit more food unto the smaller plates. Ex: She piled up her books in my living room. Ex: Why are your clothes piled up on the floor?