In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons. Here are some key features of codons to keep in mind as we move forward:
There are 616161 different codons for amino acids
Three “stop” codons mark the polypeptide as finished
One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine)
These relationships between mRNA codons and amino acids are known as the genetic code (which you can explore further in the genetic code article).

In translation, the codons of an mRNA are read in order (from the 5' end to the 3' end) by molecules called transfer RNAs, or tRNAs.
Each tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing. The other end of the tRNA carries the amino acid that's specified by the codon.

Translation: Beginning, middle, and end
A book or movie has three basic parts: a beginning, middle, and end. Translation has pretty much the same three parts, but they have fancier names: initiation, elongation, and termination.
Initiation ("beginning"): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin.
Elongation ("middle"): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.
Termination ("end"): in the last stage, the finished polypeptide is released to go and do its job in the cell.