What does a start and stop codon do?
It signals where translation should start and end.
It all has to do with how DNA stores the information to form proteins. A quick overview:
DNA are long molecules, you have about 2 to 3 meters in one cell. Only a tiny piece of this DNA is transcribed when a protein has to be produced. So the enzymes that do this have to know where a gene starts and ends, that is where the start and stop codons come in.
Every amino acid has its own specific codons = 3 bases of DNA/mRNA. The start codon always has the code AUG in mRNA and codes for the amino acid methionine. This is the signal where enzymes start transcription.
There are several stop codons (UAA, UAG and UGA) these do not code for an amino acid but only act as a signal for the enzyme to stop transcription.
So inbetween the start en stop codon is de coding region of a gene that is translated into a protein