Is it appropriate for the priest to explain what’s happening during an Extraordinary Form Mass?

Such explanations of the Mass are permitted, within reason, because the Church wants its liturgy to be understood. While it is not a good idea for a priest to do a step-by-step Mass every time he offers Mass, such supplemental explanations for children during children’s liturgies or for those who don’t ordinarily attend a particular liturgy (e.g. Eastern rite, Extraordinary Form of the Roman rite) are fine.