This is redemptive suffering – offering whatever pain you are going through, joining it to Jesus’ suffering on the cross, and offering it for the benefit of someone else, just like Jesus did.
The pain doesn’t have to be physical only, it can be worry and anguish, concern over financial difficulties, impatience with a situation, feelings of abandonement, humiliation, etc.
This comes from several scriptural passages, maybe the most famous being Colossians 1:24: “Now I rejoice in my sufferings your your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church.”
Now, Jesus’ offering was perfect, but He is the Head of the Church. We, the people, are the Body of the Church, and are called to follow Jesus in all things, including sacrifice and suffering.
There are two prayers that help me greatly with this. The first is the Morning Offering prayer, where we offer our entire day to God:
Dear Jesus, through the Immaculate Heart of Mary I offer You my day, my work, rest, prayers, joys and sufferings, in union with the Holy Sacrifice of the Mass throughout the world, for the intentions of your Most Sacred Heart, in reparation for our sins, for the intentions of our friends and family, for Our Holy Father, Pope John Paul II and (any special intentions here).
The second prayer:
O my Lord Jesus Christ, I lift up my heart to You in my suffering and ask for Your comforting help. I know that You would withhold the thorns of this life if I could attain eternal life without them. And so I commend myself to Your loving mercy, accepting this suffering in Your Name. Grant me the grace to bear it and to offer it in union with Your sufferings. Regardless of the suffering that may come my way, let me trust You always, for You are my Lord, my God, and my all. Amen.
It can also be as simple as stubbing your toe and saying “Jesus I offer this pain to you for the benefit of all souls.”