Reactions that hydrolyze the phosphodiester bonds split the DNA molecule between the phosphate groups and the hydroxyl groups of the two sugar groups.
In DNA there is a covalent bond through a phosphate group that connects the hydroxyl group (OH) at the 5' position of the pentose sugar and the hydroxyl group at the 3' position of the pentose sugar of the next nucleotide. This covalent bond is called a phosphodiester bond because chemically the phosphate group is in the diester form.
In other words, the phosphodiester bond connects the sugar in one nucleotide to the sugar in the next nucleotide, so this bond simultaneously connects the two consecutive nucleotides to form a polynucleotide chain. If there is an enzyme that catalyzes the hydrolysis of covalent bonds that combine nucleotides, what happens is that the phosphodiester link between deoxyribose sugars will break.
