Feeling "on the fence" about a subject means that you're not 100% sure how you feel about it. It means you're undecided and it can also imply that you're torn between 2 options that both good, but it's hard for you to decide which is better. in your example, "if you're feeling on the fence about the field..." Is basically saying "if you're unsure whether or not you wish to choose this field of work/study/etc, here are some reasons why you should choose it."

Being/feeling "on the fence" means that someone is between two potential decisions. For example, if i am "on the fence" about whether I will accept the invitation to go to a friend's wedding, it means I am unable to decide.

The correct expression is "sit on the fence", which means "avoid making a decision". Example: Some politicians sit on the fence about legalising abortion, to avoid losing the support of Catholic voters, who are against abortion. "Feeling on the fence" is wrong. It shows that the writer got confused about the correct form of the idiom. "The field" means "all the participants in a contest or sports". It refers to human beings and animals and should not be used as a synonym for "options". The sentence obviously does not mean humans or animals, because the preposition for humans is "with" - "stay with" - and not "stay in". "Looking at the reasons for" is exploration itself. So "explore" is superfluous because it is tautological. You can't stay in xxx because you don't have a position yet (that is what "sitting on the fence" means). So "stay in xxx" is nonsense. "Choose" is the right verb. Let's try to write this sentence in correct English: "If you are not sure which position (or option) you should take, have a look at these five reasons for choosing xxx." The writer needs to make up his mind whether he is talking about positions or options. You see how terrible the original sentence is. It is very badly written. It is very unclear. In an exam, it will score zero.

A fence is a structure, usually made of wooden boards and used as a divider. e.g. separating your garden from your neighbour's garden. If you are "sitting on the fence", then you are neither one side of the fence or the other. = undecided.