Once all have been communicated the priest reverently consumes any sacrament that remains or secures the remaining hosts in the tabernacle, after which he receives ablutions. The ablutions are the act of the priest ceremonially washes his fingers, first with wine (unconsecrated) and then with the water that was blessed and added to the chalice.
During this process he says the prayers:
Grant, O Lord, that what we have taken with our lips, we may receive with a pure heart; and that from a temporal gift it may become for us an eternal remedy.
Let thy Body, O Lord, which I have taken, and thy Blood which I have drunk, cleave unto my soul; and grant that no spot of sin may remain in me, whom this pure and holy Sacrament hath refreshed; Who livest and reignest, world without end. Amen.
The purpose of the ablutions is both practical and theological. The practical purpose is that particles of the consecrated host adhere to the fingers and these need to be removed in a reverent manner so as not to spread the consecrated elements on other things that are handled after the Eucharist. The theological purpose coincides with this, as it signifies the priest is completing his actions whereby he is interacting with consecrated and sanctified elements.
By the close of this process, as the priest is cleaning out all of the vessels, there will no longer remain any consecrated elements on the altar (barring the use of a tabernacle). As a result, from this point forward, the communicants should recognize that Christ, who was sacrificed before the foundations of the earth and who was present with them under the species of bread and wine in the Eucharist, is now in them and will be carried into the world by them. This reality causes a subtle shift in posture. Where prior to approaching the altar to receive the sacrament a communicant may have genuflected when leaving their pew, after returning to their pew they will no longer genuflect but simply bow slightly. The former act of genuflecting when leaving the pew was only done insofar as the real presence of Christ was present on the altar (or in the tabernacle); after receiving the eucharist, that real presence is now acknowledged to be inside the person.
Understanding this reality of the Eucharist helps to reinforce that we are carrying Christ into the world so that His light can shine through us into the darkness.
(as published in St. Luke's Sunday Bulletin, 9/16/18)