First of all... DNA doesn't code for protein. It codes for mRNA, which then codes for the polypeptide sequence.
Met is the start codon and goes into the A site of the anticodon of the large ribosome, and recruits tRNA for Met. Then, with the help of different proteins, the next tRNA is recruited into the P site. Met (not the tRNA, just the peptide) is transferred to Leu via peptidyl transferase and forms the chain on A. Then the elongation step. Ef-G moves the mRNA 3 basepairs down to the next codon, which transfers the chain to A site, and kicks the deacylated tRNA to the E site, or exit site. Then it repeats with the next peptide.
A deletion or addition would completely change the codons that the mRNA coded for, making a completely different polypeptide chain that wouldn't survive.
Substitution would only affect the one codon that included the substitution, because it doesn't affect the order of three of the codons.
Hopefully that helps some... And also, hopefully I didn't go too deep into it, even though I just kinda skimmed through it.