What does a betrothal mean?

According to Mark Twain's version, it means "PROMISE + KISS."

Becky, was you ever engaged?"
"What's that?"
"Why, engaged to be married."
"No."
"Would you like to?"
"I reckon so. I don't know. What is it like?"
"Like? Why it ain't like anything. You only just tell a boy you won't ever have anybody but him, ever ever ever, and then you kiss and that's all. Anybody can do it."