The basic idea is to recognize all that G-d did for the Israelites in leading us out of Egypt. Thus, we say, if G-d had led us out of Egypt, that would have been enough to make us grateful, but look at all these other things he did to help us, so we should be superthankful to Him.
Dayenu (or Dayeinu) is a song/poem that is part of the Passover Haggadah, which is recited at the Seder.
The word "dayenu" is Hebrew for "it is enough for us" or "we would have been satisfied." The song lists the miracles that G-d performed for the Jewish people, and gifts bestowed upon them, during the time of the Exodus from Egypt and immediately after. After each item, we say "dayenu."
The song follows the format "If G-d had done x and not done y, dayenu. If G-d had done y and not done z, dayenu," and so on. This detailed itemizing is an expression of gratitude for blessings received, and it helps to inculcate a mindset of thankfulness that is appropriate outside the confines of the Seder as well.
Chat with our AI personalities
The word is Hebrew, not Yiddish. (Those are two different languages.)
The word is well known to most Jews because of its repetition in every verse of a song that's sung during the Passover seder. In that context, it's understood to mean something like "plenty for us", or "sufficient for us".
Here's a metaphor of how it's used there: Mr. and Mrs. Smith hug each other one day and break into song at the realization of their good fortune, and they breathlessly exclaim to each other: "First you get promoted to Regional Manager, and then our daughter wins the Nobel Prize, and now this ... even if we hadn'twon the lottery, it would have been plenty for us !"
At the Passover seder, the song recounts miracle after miracle that the Children of Israel received from God during the Exodus from Egypt and during the next forty years until their entry into the Promised Land, saying that any one of those would have been plenty for us, would have been cause for eternal gratitude and for the establishment of a holiday to commemorate it, and how much more we have to be grateful for after having received them all.
The phrase is usually translated as "It would have been enough for us" but simply means "It is enough for us".