MRNA is made up of units called nucleotides. There are 4 different nucleotides in MRNA -- guanine, adenine, uracil and cytosine. These are most often represented by the first letter G, A, U, C. In MRNA molecule can be seen as a sequences of nucleotides.
Example:
AUGUACACUGACUAA
When the MRNA molecule is translated, the ribosome reads the order of the nucleotides. Every 3 nucleotides is called a triplet or codon. Each of these codons represents an amino acid, start order or stop order. The following is a graphic that shows the triples and corresponding amino acids
The ribosome reads the sequence beginning with the start codon (AUG) and inserts the proper amino acid. The example above would be read as follows
AUGUACACUGACUAA
AUG - Start
UAC - Cysteine
ACU - Serine
GAC - tyrosine
UAA - Stop