It's not exactly clear how the sounding of the bells keeps Aaron from dying. The point is that priesthood is sacred work, and that Aaron should never act carelessly while serving in the tabernacle. Perhaps the ringing of the bells reminds him to be intentional in all his actions while within the tabernacle. The message engraved on Aaron’s turban is a reminder that Aaron is set apart as God’s holy servant, representing the Israelites before God (that is, he bears the people’s guilt in God’s presence and bears God’s blessing back to the people).