When someone is sitting on the fence, they are not making a clear choice between two options. 想象一下一个人坐在一个篱笆上，拿不定主意。sit on the fence，表示某人态度不明确，模棱两可，无法做出鲜明的决断。

Examples:

Don’t sit on the fence when you should make a decision. 当你需要作决断的时候，不要模棱两可。

When Jane and Tom argue, it is best to sit on the fence and not make either of them angry. 当简和汤姆争吵时，最好保持中立，不要忍到其中任何一个人。