The Ten Plagues are the disasters God sent the Egyptians when Pharaoh refused to let the Hebrews go free. The plagues, which are recorded in the book of Exodus, are a demonstration of God’s power over not only Pharaoh but also over the gods of Egypt. (For instance, the first plague, in which God turns the Nile River into blood, could be interpreted as God’s sovereignty over Hapi, the god responsible for flooding the Nile.)
The ten plagues include agricultural blights, such as locusts; diseases, such as boils; supernatural or astronomical plagues, such as storms of fire or darkness; and, finally, the tenth plague — the killing of all firstborn Egyptian sons. The Jews were able to escape this plague by smearing lamb blood over their doors, reminding God to “pass over” their houses. This is the central act for which Passover is supposed to express thanks.
During the Seder, participants shake a few drops of wine out of their cups for each of the ten plagues: to remember the suffering of the Egyptians under the plagues, Jews are supposed to diminish their joy (where “joy” means “wine”). While the Plagues are God’s revenge upon Pharaoh, the Seder reminds participants to be attuned to suffering and injustice and not to take too much glee in victory.