She circumcised their son. Maybe Moses was too ill to do it. Zipporah must have realized what was wrong, or Moses may have told her: their son was not circumcised yet. Maybe it refers to their oldest son, because in Exodus 4:22–23 the firstborn sons are mentioned. Maybe it refers to the youngest son (Eliezer). Moses did circumcise Gershom, but Zipporah did not agree so they did not circumcise Eliezer. Now she does agree, but she is angry and said, a bridegroom of blood.
24 At a lodging place on the way the LORD met him and sought to put him to death.