Our parish is blessed to have a very young, newly ordained priest assigned to us for a couple of months now.
Yesterday at Mass, he reminded the congregation that during the Confiteor, the rubrics in The Order of Mass state we strike out breast during the words "through my fault, through my fault, through my most grievous fault" (mea culpa, mea culpa, mea maxima culpa").
The USCCB commentary on The Order of Mass states
"During the Confiteor, the faithful should “strike their breast” while saying, “through my fault, through my fault, through my most grievous fault.” This prescribed “striking” is a symbolic tapping of the chest with a clenched fist over one’s heart, signifying remorse. This is part of the beauty of our Catholic liturgy – sacramental words are complemented by sacramental actions. This action also recalls the penitent tax collector in Luke, chapter 18, who “beat his breast and prayed, ‘O God, be merciful to me a sinner.’”
I actually like an older explanation even better. St. Augustine, speaking of striking the breast said
"What does this mean except that you wish to bring to light what is concealed in the breast, and by this act to cleanse your hidden sins?"