The fluid mosaic model describes the structure of cell membranes, illustrating how they are flexible and dynamic. According to this model, the membrane is composed of a double layer of phospholipids, with proteins embedded within it. These proteins can move laterally, creating a "mosaic" appearance, while the fluid nature allows for the movement of various molecules across the membrane. This model emphasizes that the cell membrane is not a rigid structure but rather a semi-permeable barrier that regulates the entry and exit of substances. The fluid mosaic model is essential for understanding cellular processes, including communication and transport, and is foundational in the study of cell biology.