What is the meant by the following statement the cell membrane is said to be "fluid"?

A cell membrane is said to be ''fluid'' as it is a semi permeable substance, so that organelles of the cell can move.

Individual lipid and protein molecules are free to move laterally in the plane of the membrane.

Explanation:

Cell membrane is the lipid bilayer and is made of two layer of phospholipids in which proteins are embedded. The cell membrane is fluid since most of the interactions among proteins and lipids as well as between various phospholipids molecules of the membrane are non-covalent. These interactions allow the individual lipid and protein molecules free to move laterally in the plane of the membrane.