An "Heroic Act of Charity" is the offering of the satisfactory value (not the merits) of all of our sufferings and works of our rest of our lives and of any time we may spend in Purgatory for the relief of the souls in Purgatory. We do this by first deciding to do so, and then praying (using our own words or the more formal prayer below) to offer these things to God through Mary's hands.
Doing this is not a matter of taking a vow; it doesn't bind under pain of sin, and it is revokable (unless one vows never to revoke the Act). But it is a tremendous sacrifice, hence the name. It is truly heroic, a giving up of one's own earned relief from the temporal effects of sin -- even relief of the sufferings of Purgatory -- for the good of others.
In addition to asking God to use their satisfactory works for the souls in Purgatory, those who make this Act also receive a plenary indulgence (under the usual conditions) for the souls in Purgatory each time they receive Communion, and each time they hear Mass on Mondays for the sake of the departed. Words to a formal Act of Heroic Charity are as follows:
O Holy and Adorable Trinity, desiring to aid in the relief and release of the Holy Souls in Purgatory, through my devotion to the Blessed Virgin Mary, I cede and renounce, on behalf of these souls, all the satisfactory part of my works, and all the suffrages which may be given to me after my death. In their entirety, I offer them to Mary, the Most Holy Mother of God, that she may use them, according to her good pleasure, for those souls of the faithful departed whom she desires to alleviate their suffering. O my God, deign to accept and bless my offering which I make to Thee through the most august Queen of Heaven and Earth. Amen.