It is a song that celebrates the Hebrews escape from slavery in Egypt and their recognition and gratitute to God for His role in that.
Throughtout the song, they say, if only this had happeed, it would have been enough. But that and more happened, so each line gives another thing they are grateful for. It also means that although they are grateful, they desire more, if that is God's will for them.
Passover, and this song, are celebrated present tense. The liberation of the Jewish people is not something that happened only thousands of years ago. What happened to their ancestors, also happened to those alive now, they are one with their ancestors in this experience. It also continues to happen, the liberation is never ending.
While they are gratelful for what God has provided, and that would be enough, they desire more, and because God gives them more at every step of the way, it is their duty to work for more.
A part of the song:
If only He had brought us out of Egypt, that would have been enough.
If only He had split the sea for us, that would have been enough.
If only He had provided for our needs in the wilderness for 40 years, that would have been enough.
If only He had given us the Torah, that would have been enough
In our modern age we might want to continue:
If only He had end slavery, that would have been enough.
If only He had given minorieties and women the right to vote, that would have been enough.
If only He had given us the right to strive for a living wage, that would have been enough.
The Dayenu rightfully pays respect to that which we should be thankful for, and it also reminds us that there is always another step on the road to social equality and it is our responsibility to take that step.