Cells contain various structures to perform certain functions. Parts of a basic cell include the Cell Wall, Cytoplasm, Ribosomes, and Nucleus. Animal cells differ in important ways from Plant cells, and this question refers to a structure found only in plants.
The most important function of a plant cell is Photosynthesis, the mechanism by which sunlight is transformed into energy and nutrients within the plant. To process sunlight, plant cells have structures inside called Thylakoids. These allow photons to be absorbed from sunlight and passed along an Electron Transport Chain to break water molecules down and and provide to energy needed to create ATP, the molecule that transfers energy from cell to cell.
The Thylakoid is membrane-bound, which means it has an organic membrane surrounding the entire structure, and is typically embedded in the Chloroplast Stroma. Inside, it contains a Lumen, which absorbs protons during the Light-Dependent Reaction. Thylakoids often group together into stacks, or Grana, connected by Thylakoid Stroma, a thick fluid used for carbohydrate production. Stacks of Thylakoids are found inside Chloroplasts, and inside Cyanobacteria.