When this sequence begins transcription, the RNA comes of the DNA and ribosomes hop onto it to start translation. However, if there is little histidine in the cell, the ribosome stalls because there are no aminoacyl tRNA's that are charged with histidine. This leaves a long stretch of RNA (for RNA polymerase is still transcribing it) with no ribosomes bound to it. The sequence of this RNA allows it to form a terminator loop only when ribosomes are bound to it, at which point the RNA is cleaved and the RNA polymerase stops transcribing the genes. Thus, the terminator only functions when the ribosome is not stalled; that is, when there is already plenty of histidine in the cell. The site at which the potential terminator loop forms is called the attenuation site.
Note that many amino acid synthetic operons are also controlled by some form of attenuation. The tryptophan operon has attenuation control as well as the repressor control.