Fence sitting, sitting on the fence, sitting on a fence, are all terms for someone who refuses to make a decision. Usually it describes someone who will not take a stand on two very defined and clearly opposing options/opinions. This can be both good and bad.
Clair was angry that Mandy occasionally used drugs; Mandy thought drug use should be legal. Angela was neither for nor against it, and sat on the fence about it, instead trying to calm her friends down.
The politician, when questioned on abortion, sat on the fence, never giving a clear opinion; he was afraid of losing support from the right or the left.