When a C=C bond is next to an oxo (or other double bond =R which can form a negatively charged R) group, a nucleophile can attack at the C=C instead of the C=O via. a conjugate addition. The mechanism follows two steps: addition followed by protonation.
The p-system in these reagents is conjugated, and so reacts differently to the way in which the isolated functional groups would.
Addition to the C=C is termed '1,4-addition', addition to the C=O is termed '1,2-addition'. Many factors effect which reaction course is followed, including reaction conditions, structural factors and the nature of the nucleophile itself.