The Minhag is when the Cohanim say the last three words of Birkat Cohanim on Yom Tov the Kahal says a few sentences and their contents are about dreams. What is the connection between dreams and Birkat Cohanim?
The Talmud, tractate Brachot 55; b, states that when someone dreamed something he can't interpret, he can say this prayer during the Birkat Cohanim and ask Hashem to turn the outcome of this dream for the good. This is said during the last word of each of the three Berachot of the Cohanim, (some have the custom that the Cohanim sing before the last word to accommodate those who say this prayer.)(Mishna Brura 130; 3) Some add the verse – and may the blessing of the Cohanim be upon me. (ibid 4) The Birkat Cohanim is a powerful blessing that can turn bad into good and that is the purpose of the prayer for the dreams. This is all on an esoteric and Kabbalistic level.