Starch is a polysaccharide: basically, a sequence of polymer molecules “glued together” by their carbon atoms. Also in these molecules are oxygen atoms, which bind to hydrogen to form one of the simplest structures in organic chemistry, called a hydroxyl group. Ethers are also molecular structures called alkyl or aryl groups, which bind to oxygen atoms in the starch to form a starch ether.
The structure of a starch ether is “hydrophilic”, meaning it attracts and retains water. This is how starch ethers create benefits across a range of industrial products: keeping water in a compound lets manufacturers control how a mixture thickens, emulsifies, and stays stable in a range of conditions.