Steric Effects
The SN2 reaction is very sensitive to steric hindrance because, in the TS, the reacting carbon is a trigonal bipyramid where there are five groups in close proximity. The volume of the R groups critically determines the energy of the TS and hence the reaction rate.
What can you deduce from the table?
The increase in the number of substituents at the reacting carbon causes a drastic decrease in the reaction rate.
The linear lengthening of the chain of one of the substituents of the reacting carbon only slightly decreases the reaction rate.
In contrast, branching of one of the substituent chains at the position next to the reacting centre severely decreases the reaction rate.