could someone elaborate on what a retron is in terms of retrosynthesis and how it differs from a synthon?
I'm not sure how standardized the terms "retron" and "synthon" are in industry or academia, but here's how I understand them:
"Retron," coined by E J Corey, can be used to refer to a particular portion of a molecule which indicates what retrosynthetic "transform" (the opposite of a forward-direction synthetic reaction) can be used to reduce structural complexity. So for example, ethyl acetoacetate exhibits the retron (ie, the 1,3-dicarbonyl) for a Claisen condensation transform.
"Synthon" is used in the "disconnection method" of retrosynthesis (cf. Stuart Warren's book), and it refers to the imaginary fragments that result from breaking a bond heterolytically. The synthons which result from disconnecting ethyl acetoacetate at the bond between carbon 2 and 3 and giving both electrons to carbon 2 are an acylium cation and an enolate anion. These synthons are replaced by "synthetic equivalents" or "precursors" in the forward synthesis, namely two ethyl acetate molecules.