Here's a breakdown:

* Thylakoids: These are flattened, sac-like structures that are stacked into columns called grana.

* Thylakoid membrane: This is the membrane that surrounds the thylakoid. It contains chlorophyll and other pigments necessary for photosynthesis.

* Lumen: The space inside the thylakoid is called the lumen.

The thylakoid membrane plays a crucial role in photosynthesis. It is where light-dependent reactions take place, converting light energy into chemical energy.