Esterification is the reacting of a carboxylic acid with an alcohol to produce an ester. In order for this reaction to work, two things are required: removal of the water produced as a byproduct, and an acid catalyst. Sulfuric acid is a fine source of protons as an acid catalyst. When the alcohol hydroxyl group attacks the carbon of the carboxylic acid, a species called the tetrahedral intermediate initially forms. A proton from the sulfuric acid helps to promote the formation of this intermediate species. Once this intermediate is formed, it collapses to eliminate a water molecule and leave the newly formed ester product behind. So the acid is needed as a catalyst to help the tetrahedral intermediate to form.