The four nucleic bases are what makes up the code of the DNA.
The essence of DNA is a complex information code. The four bases in combinations of three code for all of the information necessary for making the proteins that form living things. Besides coding for the actual proteins the DNA also codes for how the proteins are to be assembled and regulated.
The information coded for in the DNA is more complex than any computer program yet written by people. All of that information is recorded in the nucleic bases. This makes the nucleic bases the chemical building blocks of DNA.