Nonsense mediated decay is a pathway used to induce the degradation of mRNAs with premature STOP codons (which are caused by nonsense mutation and alternative splicing patters (giving non-productive products)).
During translation, the ribosome comes into contact with a STOP codon, however the mRNP complex at the poly(A) tail of the 3’ UTR (including the poly(A) binding protein) is not contacting the ribosome. This alerts the ribosome to the premature STOP codon, inducing the decay pathway. This leads to the rapid degradation of the mRNA.