Ambident nucleophiles are nucleophiles that are capable of attacking the substrate (alkyl halide) through two different atoms.
It so happens due to the presence of two nucleophilic centres which arise from the contributing (resonance) structures thatare possible for the ion.
e.g., In NO2– ion, there is a lone pair of electrons on N and therefore makes it nucleophilic while oxygen by virtue of the negative charge acts as a nucleophile. Thus, NO2– can attack via O or N atom thereby making it ambidentate.