You bet 是肯定，一点也没错的意思。
举几句口语记忆下：
You bet it is 一定是这样
They won't make it ,You bet .
你肯定办不成
will you tell her ?You bet
你会告诉她吗?当然会 这里You bet=I will
通过bet 这个词我们来研究下相关的短语学习
bet on 就...打赌
He bet on that horse
他把堵住压在那匹马上
make a bet 打赌
if you don't believe ,let's make a bet
如果你不相信，咱们就打赌。
打个小赌可以说：make a little bet
How about make a little bet ?
我们打个小赌如何?