And [the hem with bells] shall be on Aaron when he ministers, and its sound shall be heard when he goes into the Holy Place before YHWH, and when he comes out, so that he does not die.
Exodus 28:35

There's a popular myth that the purpose of these bells was so the other priests could hear if the high priest dies while in the Holy of Holies, they'll hear the bells stop and drag him out by a rope tied around his ankle. However, the High Priest didn't wear this hem in the Holy of Holies, only in the Holy Place, where the other priests could see him plainly.