_Monomer_ in this context means the protein has only one polypeptide chain. In some proteins, after the individual chains have folded into their 3D or tertiary structure, they associate (generally non-covalently) into a higher order or quaternary structure. A protein with a quaternary structure containing two copies of the same chain would be called a homo-dimer. One with one copy of each of two different chains is a hetero-dimer.
Haemoglobin is a well-known example of a protein with quaternary structure — it has two copies of the alpha- and two of the beta- chain. Further elementary treatment can be found in Berg _et al._ and explanation of the nomenclature in this Wikipedia page.
Monomeric _polypeptide_ is somewhat misleading in the sentence quoted. Monomeric _protein_ would have probably been better as you wouldn't be likely to say ‘dimeric polypeptide’, given that polypeptide implies a single chain.