Tertiary alkyl halides are practically inert to substitution by $S_{N}2$ mechanism because of:

In an $S_{N}2$ reaction, the nucleophile attacks from the backside of the carbon-halogen bond.In tertiary alkyl halides, the central carbon is surrounded by three bulky alkyl groups.This creates massive steric hindrance, preventing the nucleophile from getting close enough to the carbon to form a bond.