Because the most electrophilic site of an alcohol is the hydroxyl proton and because OH- is a poor leaving group, alcohols do not undergo substitution reactions with nucleophiles. However, they can be converted into tosylates or other sulfate esters, which have very good (sulfate) leaving groups that are readily displaced in substitution reactions.
Nucleophiles that displace tosylate groups include hydride ions, H-. Therefore, tosylates formed from alcohols undergo nucleophilic substitution reacctions with hydride sources, such as lithium aluminum hydride (\(LiAlH_4\), aka \(LAH\)). The net result of the process is the reduction of alcohols to alkanes.
\[ROH + TsCl \rightarrow ROTs\]
\[ROTs + LiAlH_4 \rightarrow RH\]