At Mass the last ritual action of the Preparation of the Gifts is the “hand washing” of the priest-celebrant. Washing one’s hands is a pretty basic human action, something which usually is done for one of two reasons.
The first reason to wash one’s hands, of course, is if they’re dirty. And this is not why the priest washes his hands at Mass! (At least, it’s not supposed to be: liturgical law actually instructs the celebrant also to wash his hands before Mass for this purpose! Plus, if anyone’s hands are soiled during the service – if holy oils are used, for example – another hand-washing takes place then and there!)
No, the ritual washing of hands at Mass takes place for the second reason: we cleanse our hands – and indeed our entire self – to show respect for what we will touch next. In the Western Catholic Church, the “standard” hand-washing takes place right before the great Eucharistic Prayer, during which the bread and wine become Christ’s Body and Blood. The next time the priest touches the gifts, they will be transformed! (In the Eastern liturgies of the Church, the hand-washing occurs right before the priest receives the gifts from the people at the beginning of this part of Mass. In a way that nicely emphasizes how the people and what they give are holy things too!)
Finally, remember that the priest is supposed to represent everyone in his actions. So before I “wash my hands” at Mass, please do your best to “cleanse your soul” as well!