Proteins are polymers because they consist of long sequences of amino acids. Lipids have a single structure formed by glycerol and three fatty acids.
A monomer is a molecule that can attach itself to other monomers, thus forming a more complex structure. A large molecule that is composed of two or more monomers is called a polymer.
For example, a glucose molecule is a monomer. When two glucose molecules are attached, they form a true polymer called maltose.
One important feature of polymers is hydrolysis, which means that the bonds between monomers break in the water, making the substance soluble. The bonds between glycerol and fatty acids, as well as the sequence of molecules in lipids, is different from monomers. Because of this, lipids are not soluble in water and thus are not considered true polymers.