What is an allegory?

One is literal and the other is symbolic.

Galatians 4:23-31
23But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.

24Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.

25For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.

26But Jerusalem which is above is free, which is the mother of us all.

27For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband.

28Now we, brethren, as Isaac was, are the children of promise.

29But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.

30Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.

31So then, brethren, we are not children of the bondwoman, but of the free.

Agar is in bondage to this day- (literal)

We as Issac was are the children of promise. Issac is symbolic of the church.