What are secondary valence bonds?

A secondary bond is defined by an attraction between molecules, however no sharing or transfer of electrons (as with a primary bond).

Secondary bonds are weaker than primary bonds, and their effects are often overlooked because of the more powerful strength of the primary bond.

To learn more about the difference between primary and secondary bonds, I'd recommend reading this related Blurtit answer: //business-finance.blurtit.com/107504/what-are-primary-and-secondary-valence-bonds

You might also find this extra reading material from the University of New South Wales helpful: www.hsctut.materials.unsw.edu.au/Bonding/bonding3a.htm