Allegory: a metaphor in which a character, place or event is used to deliver a broader message about real-world issues and occurrences. Allegory (in the sense of the practice and use) can readily illustrate or convey complex ideas and concepts in ways that are comprehensible or striking to its viewers, readers, or listeners.
Galatians tells us in vs verse 24 Sarah represents the Spirit and Hagar represents the flesh. The two women allegorically represent the two covenants, the son born of the flesh and the son born of the free woman through the promise. The battle of the flesh will continue to rage until our last breath.
See how Paul uses an allegory in Galatians 4:21-29 to teach us a complex idea in a way we can grasp the truth of Sarah and Hagar, and how they represent us.