Genetic coding on mRNA is the sequence of base triplets, each triplet specifying or coding for a particular amino acid. The mRNA was transcribed from a section of DNA that also contains this complementary coding sequence. Three bases make up a triplet that codes for a particular amino acid. Transfer RNA will carry the particular amino acid to mRNA in the translation process, where ploypeptides or proteins are synthesized. (Simply expressed, genetic coding is method by which information encoded within genetic material (DNA or mRNA) is translated into proteins by living cells.)
It likes a combination lock, which need a right code (a combination of numbers) to be open. Same thing, the right code ( a combination of gene building units) is needed to constitute a right gene to produce a right enzyme/protein for our needs. A set of wrong numbers cannot open a lock, and a set of wrong building units cannot make a right protein, which leads to mutation or diseases.