The book of Exodus is the story of God rescuing the children of Israel from Egypt and forging a special relationship with them. Exodus is the second book of the Pentateuch (the five books of Moses), and it’s where we find the stories of the Ten Plagues, the first Passover, the parting of the Red Sea, and the Ten Commandments.
The book gets its name from the nation of Israel’s mass emigration from Egypt, but that’s only the first part of the story. This book follows Israel out of Egypt into the desert, where the nation is specifically aligned with God (as opposed to the idols of Egypt and the surrounding nations). This is the book in which God first lays out his expectations for the people of Israel—we know these expectations as the 10 Commandments. Most of the Old Testament is about how Israel meets (or fails to meet) these expectations. So if you want to understand any other book of the Old Testament, you’ll need a basic understanding of what happens in Exodus.