Like the comment above mine, "I guess" is a way of agreeing to something, but not whole-heartedly. It's more like sort-of saying that you agree with something.
Let us say you don't really like hamburgers, but your whole family does, and they suggest going to get hamburgers for dinner. They ask you if you want to get them, and, since you don't want to make a problem out of it, you respond with "I guess..." It doesn't mean no, but it doesn't mean yes, it's in the middle.
"I bet" is a phrase that can be used to agree with a situation. Here's an example conversation:
"I tripped and broke my ankle!"
"Yeah, and it really hurt."
-"Ouch, I bet"
So "I bet" is kind of saying you agree and that something obviously happened, like how breaking your ankle would obviously hurt.
It's pretty confusing and honestly, I don't typically use or hear lots of people saying "I bet" on an everyday basis, so if you don't completely understand its fine and there are many alternatives to the phrase. Nonetheless, I hope this helped!