Substitution: Nucleophilic @ Acyl-Nfg
Many acyl functions (for example acid chlorides, R-COCl, and to a lesser extent esters, R-COOR) are highly susceptible to attack by nucleophiles. According to resonance theory, the adjacent carbonyl function stabilises the reaction transition state by delocalising any net negative charge onto the oxygen.

An alternative view is that the oxygen polarises the adjacent carbon (with respect to the sp3 analog) which increases susceptibility to Nu attack.

Confusingly, this sequence is also known as the "tetrahedral" mechanism and as "addition-followed-by-elimination".

For more information look in the Chemogenesis webbook sections on substitution reactions and substitution, transfer, abstraction, displacement (STAD).