An interdict is a prohibition of religious services and rites, in which only baptisms and last rites are allowed to be performed. Pope Innocent III threatened European rulers with an interdict on no less than 85 separate occasions during his reign from 1198 to 1216. This, alongside excommunication, was a spiritual weapon that he wielded to force rulers to bend to his will and was used most famously by Innocent III in England from 1208 to 1214. For these six years, not a single person in England attended church and received the sacraments or got married.
The reason why this was such an effective political weapon lies in the medieval belief in salvation. People in the Middle Ages believed that they could only receive salvation and go to heaven if they received grace. In order to receive this, they had to attend church and receive the sacraments, do penance and be absolved and have their marital unions blessed by their parish priests. In short, these rites guaranteed a place in heaven and, if they were taken away, put a person on the road to hell. For the typical medieval person, the thought of going to hell was terrifying and Innocent III knew this. He knew that this threat was so devastating that it was only a matter of time before a monarch would submit to his authority - to save the souls of his subjects, if nothing else.