What's "all of a sudden" mean?

Native speaker’s answer
Rebecca
"All of sudden" means "suddenly" or "immediately;" quickly and unexpectedly. It has a nice conversational tone for stories and brings a lot of emphasis to something. Ex: I crossed the street, and all of a sudden, I saw a bear in the road. Ex: I sat in the dentist's chair, and they injected me, then all of a sudden I woke up, and the procedure was done. Ex: All of a sudden, he turned around and saw me.