Explanation Needed

Yesterday I was puzzled by the prayer after communion at Mass. I'm still puzzled. Can someone explain this one to me?

"Lord, hear the prayers of those who receive the sacraments of eternal salvation. As we honour the compassionate love of the Virgin Mary, may we make up in our own lives whatever is lacking in the sufferings of Christ for the good of the Church."

Can someone explain to me how the sufferings of Christ are lacking? I'm confused.