What is Disconnection and synthons?

A disconnection is the imaginary cutting of a bond of the target, the reverse of a bond-forming reaction. Cutting a bond heterolytically gives two synthons, idealised charged fragments, one a donor (nucleophilic, often negative), the other an acceptor (electrophilic, often positive). A synthetic equivalent is a real reagent that behaves as a synthon: a Grignard reagent for the carbanion $\ce{R-}$, an aldehyde for the cation $\mathrm{R\overset{+}{C}H{-}OH}$.