Nonsense mutations or stop mutations are alterations in the DNA that, when transcribed into mRNA, introduce a premature translation termination codon. This change halts the ribosomal translation process at an earlier site than normal, producing a truncated, non-functional protein. In other words when the protein is on its way to be formed this mutation commands the protein to stop. At this time the protein is not fully formed. The function of PTC124 is to jump this stop sign and eventually, a full length of protein is produced.
If you have an X at the end of your mutation (i.e R553x) then you have a nonsense mutation.