The Tashlich Casting ritual is performed on Rosh Hashanah afternoon. Little crumbs of bread are tossed into water, expressing the Torah verse: "You will cast all their sins into the depths of the sea."
Some also shake out the hems of their clothing, or Tallit Katan, three times, to symbolize that we put our very heart into the casting out of sins, to personally cleanse ourselves of sin. There is support for this custom in Scripture (Nehemiah 5:13): "I shook out my lap, and said: 'So may G-d shake out every man from his house (who does not fulfill this promise)' " [Mahzor Ohole Yaakov].
The Tashlich ritual is usually performed at a sea or river that contains fish. If there is no such water nearby, the ritual is done at a spring or fountain.
In Jerusalem, Tashlich is said near cisterns that hold water. The inhabitants of Safed go up to the roofs of their houses from where they can see Lake Kinneret, and say the prayer there. In Aden, the ceremony is conducted near the ritual bathhouse in the courtyard of the main House of Prayer.
Prayers beseeching mercy Prayer Verses are said, including a reference to water: "They shall not hurt nor destroy My Holy mountain; for the earth shall be full of the knowledge of the Lord, as the waters cover the sea" (Isaiah 11:9). Then we add a prayer composed by Rabbi Hayyim Joseph David Azulai (18th century) to arouse compassion for Israel, and ask G-d to remember and inscribe us in the Book of Life.
The waters also recall the many centuries that Jews have been killed, burned to death and drowned in water for the sanctification of his Name. We ask G-d to return his Presence to Zion and return Israel to their land.
Other Interpretations
We go to a sea or rivers on Rosh Hashanah, because the Midrash relates that when Abraham, went to bind Isaac his son, Satan tried to stop him and obstruct his way by turning into a great river, until the waters reached up to Abraham's neck. Abraham raised his eyes to heaven and cried out: "Master of the universe, you have chosen me, and promised, "You shall proclaim my Name to the world", and now, "The waters threaten my life" (Psalm 69:2). If I, or Isaac my son, drown, who will proclaim your Name to be One?"
Said G-d: "By your life, my Name shall be declared One in my world only through you." At once G-d rebuked the river,it receded and Abraham was saved. (According to Maharal)
It is customary to go to a river with fish, because we are compared to fish caught in the net of divine judgment. Gazing at the river, we contemplate repentance.(Lvush)
Another reason: We visit rivers with fish, that the evil eye may not govern us, as it cannot govern fish, and that we may be fruitful and multiply like fish. Just as the fish eyes are always open, so let the Divine Eye always be open to protect us.
The custom of going to a body of water on Rosh Hashanah is symbolic, for the waters which are now at this place were not here before and will not remain afterward. So, the sinner says to himself, "I will not repeat my sin," the sin, like the waters, will move on. (Sefer ha-Hayyim)
We should avoid a superficial light-minded attitude: "Hey, here I will shake away all my transgressions," thinking we can just shake away transgressions. This is really desecrating G-d's name. When we observe the custom, we say, "I will perform the Casting." The purpose of the custom is a prayer that G-d cast our iniquities into the depths of the sea, and sincerely repent.
Adapted from Days of Awe by Nobel Prize winner Shai Agnon