I bet = I'm pretty sure

I bet can mean that you are willing to give something away if an event doesn't happen the way you want. However, there is also a colloquial meaning. It can also be used for sarcasm. Here's an example: "I have an interview today. I think I have a good chance of getting the job!" Then another person says, "Yeah. I bet." This means that the second person doubts that the first person will actually get the job. Therefore, "I bet." can mean "I doubt it." It is somewhat mean, but many people use it anyway with friends and family.

I risk money for a race or other thing.