Dayenu means “it would have been enough”. When we were slaves in mitzrayim, Hashem set us free. For us being free would have been enough, but instead Hashem did extra and took us out of Mitzrayim with many miracles. When Moshe was leading the Jews out of Mitzrayim, they hit a sea, and Hashem made a miracle for the sea to split and all the Jews made it through to dry land. When the Egyptians went to chase after the Jews, Hashem made another miracle that the sea went back to how it was and drowned all the Egyptians. The song Dayenu is representing how grateful we are to Hashem for all that Hashem has done for us. At this point of the seder, we are in the middle of Maggid.  During Dayenu some people have a custom to hit each other with scallions to remind us of how we were slaves in Mitzrayim and how Hashem helped us. This teaches us that Hashem is our father and he does a lot for us. If we weren't taken out of Mitzrayim we would be working for the Egyptians and we would be going through a difficult time, but Hashem saved us and gave us the Torah. 

Citations:


haggadah Section: -- Cup #2 & Dayenu