Phospho-' refers to the element phosphorus, '-di-' means two, and '-ester' refers to a specific type of chemical bond that includes a single atom bound to two oxygen atoms. So, a phosphodiester bond is a phosphorus atom involved in two ester bonds.This bond is a key structural feature of the backbone of DNA and RNA and links the 3’ carbon of one nucleotide to the 5’ carbon of another to produce the strands of DNA and RNA.