24At a lodging place on the way, the LORD met Moses and was about to kill him. 25But Zipporah took a flint knife, cut off her son's foreskin and touched Moses' feet with it. "Surely you are a bridegroom of blood to me," she said. 26So the LORD let him alone. (At that time she said "bridegroom of blood," referring to circumcision.)This event happens right before Moses goes to Pharaoh to deliver the Israelites from Egypt. This account of Zipporah circumcising her son, touching Moses' feet with the foreskin, and then calling Moses a "bridegroom of blood" seems very odd. On top of that, we read that God was about to kill Moses and the actions of his quick-thinking wife saved his life. This also seems out of place.
There is a fair amount of consensus among Old Testament scholars that this is a story about the profound importance of circumcision in the covenant. Circumcision was a kind of signature on the part of a person, which meant they wanted to be in a covenant relationship with God. Apparently Moses had not circumcised his son, although he knew he should have. Most likely, he has not been circumcised himself, which means he is deliberately disobeying God. He is holding himself outside of a covenant relationship with the Lord. There is simply no way he can take on God's mission until this is remedied.
So Zipporah, his wife, recognizing what's happening, takes a flint knife--the appropriate instrument--and circumcises their son. Then she touches Moses' feet. The word for feet is often a euphemism in Semitic culture for genitals. She touches him as a kind of temporary, vicarious circumcision of Moses until the time comes when Moses can be properly circumcised.
When Zipporah makes that statement about "you're a bridegroom of blood to me, " it might sound like a negative thing to say, like, "What a lot of bloodshed you and your God are causing me." But this is not what she is saying. It was actually a ritual marriage statement that a Hebrew bride would make when her groom had been circumcised before marriage. It's a positive statement: "You are blood kin to me. We're members of one family. We're a covenant people together." What's really happening here is that Moses has been disobedient to God's covenant calling. God is serious about it, and Zipporah's quick insight and actions save Moses' life!