When someone is supposed to choose between two things, but hasn't made a choice yet, you can describe them as "on the fence". Imagine that the two choices are areas that are separated by a fence, and someone is sitting on top of the fence between them.
You can be "on the fence" about a life decision, such as where to go to college:
She's still on the fence between Duke and Yale.
Or you can use "on the fence" to talk about someone's opinions on important issues such as politics or religion:
A: How do you feel about the war?
B: You know, I'm kind of on the fence.
The phrase "on the fence" doesn't really carry a positive or negative association, but some people don't like it when people are "on the fence" on important issues. They feel that people should clearly choose a side and stick with it. Other people think that it's important to understand both sides of an issue.