Tashlih - תַּשְׁלִיךְ

On Rosh HaShana, there is a custom to go to a near water source (pond, river, ocean, etc.), and to do Tashlih (symbolic throwing away) of all your sins into the water. So if you have never seen this act before, and you see a crowd of people emptying their pockets into the water on this day, don't be alarmed. It is just a prayer.