Here's a breakdown:

* Codons are the three-nucleotide units within messenger RNA (mRNA) that specify which amino acid should be added next during protein synthesis.

* RNA (ribonucleic acid) is a single-stranded nucleic acid that plays a crucial role in protein synthesis.

* Nucleotides are the building blocks of RNA, composed of a sugar (ribose), a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and uracil (U).

Therefore, a codon is a sequence of three consecutive nucleotides in mRNA that codes for a specific amino acid.