Why is the model of the structure of biological membranes described as ‘fluid mosaic’?

The biological membrane is a complex, heterogenous structure, built by two lipid layers with proteins and molecules immersed and in it. Its properties were developed due to the fact that its main function is to separate and therefore protect the inside of the cell from the outer environment. The model of biological membrane described as "fluid mosaic" states that the molecules that are immersed within the lipid bilayer can move, to some extent, from one spot in the bilayer to another, depending on the current life situation of the cell (what is the exterior environment of the cell at the given moment? How is the cell supposed to respond and interact with it? What is the current metabolic state of the processes ongoing inside the cell?), whereas other, mostly proteins, rather remain stable in one spot of the bilayer throughout the cell's life cycle. Such "behaviour" of a cell membrane offers both stability of the cell structure thanks to the more "stable" elements, as well as provides for flexibility in order to interact with inner and outer cell environment in an effective way.