mRNA is processed by a ribosome and other RNA, and translated into a polypeptide.

mRNA originally comes from DNA via the process of transcription.

3 consecutive bases of mRNA will code for a specific amino acid using the genetic code. There are 4^3 (64) possible combinations, but only 20 amino acids, so some of these map into the same amino acid. This means the genetic code is degenerate.

A sequence of 3 bases represents a codon.