What's he mean by "I bet" and when's it used?

Native speaker’s answer
Rebecca
"I bet" is an informal phrase used to indicate one's total agreement with something. For example, "I'll bet you did" is used to share the speaker's total agreement or belief. Ex: A: I loved the movie. B: I bet you did, the lead is your favorite actor. Ex: A: The concert was amazing! B: I bet you enjoyed it, I heard it was amazing.