Dear student
SN2 reaction takes place in a concerted (single step)manner.Unlike SN1, here leaving group is still intact with the substrate so nucleophile can not attack from the same side as the leaving group. Hence nucleophile attacks the substrate from the opposite side of the leaving group leading to "Walden inversion" at chiral centre.Tertiary halides never reacts in a SN2 manner because site of attack is sterically hindered.
Walden inversion is the inversion of a chiral center in a molecule in a chemical reaction. Since a molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other. For example, R will convert into S after reaction, wedge will convert into dash