What is meant by the "fluid-mosaic" model of cell membrane structure?

The cell membrane consists of a double layer of phospholipids, with proteins floating in it. Phospholipids are a type of molecule with a phosphate head, and fatty tails, which form a double layer around the cell. The proteins sit within the membrane at intervals, which is why the cell membrane is a "mosaic". It is described as "fluid" because the proteins can move around within the membrane, by moving between the phospholipids.