Paul begins a story to illustrate a truth—that law and grace cannot exist together.
“This story of the two sons is like a picture of what I will tell you now”
An “allegory” is a story in which the people and things in it represent other things. In Paul’s allegory, the two women referred to in Galatians 4:22 represent two covenants.
“Mount Sinai” here is a synecdoche for the law that Moses gave to the Israelites there. AT: “Mount Sinai, where Moses gave the law to Israel” (See: rc://en/ta/man/translate/figs-synecdoche)
Paul treats the law as if it were a person. AT: “The people under this covenant are like slaves who have to obey the law” (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-personification)
“is a picture of”
Hagar is a slave and her children are slaves with her. AT: “Jerusalem, like Hagar, is a slave, and her children are slaves with her” (See: rc://en/ta/man/translate/figs-metaphor)