This is a tRNA molecule which suppresses amber mutations because it has mutated to recognize the amber codon UAG (which normally signals that the translation of mRNA into an amino acid chain should stop) as a signal for inserting whatever amino acid it carries into the chain.
As a result, it can prevent the amino acid chain from ending before it is completed.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.