The reaction of benzene with an alkyl chloride and aluminum trichloride is called a Friedel-Crafts alkylation. The aluminum trichloride is a lewis acid that reacts with the alkyl chloride, and this converts the alkyl group into a very good electrophile. The overall reaction is an electrophilic aromatic substitution. A pi bond of the benzene ring will attack the electrophile as a nucleophile and form a new carbon-carbon bond. The result is a monosubstituted isopropyl group on the benzene ring.
However, we are told that the conditions are set up to encourage dialkylation. The isopropyl group is a group that donates electron density through induction. Therefore, it is an ortho- and para-directing group. This means that once one isopropyl group is substituted on to the benzene ring, the second substitution will be favoured in the ortho- and para-positions. In this instance, the major product is the 1,4-disubstituted product. The reason being is that the isopropyl groups are very bulky, and the steric hindrance of having two isopropyl groups adjacent leads to a small amount of this product being formed.
There will also be trace amounts of the meta-substituted product, but the 1,4- and 1,2-substituted products are more favoured.