On the fence means still undecided between the two sides. Not committed, and often not wanting to be.
The image is of somebody sitting on a boundary who could drop either way. It nearly always follows sitting, and sitting on the fence is a criticism, since it accuses somebody of avoiding a choice rather than genuinely weighing it. A fence-sitter is the noun. Politics uses it heavily. Coming off the fence means finally declaring which side you are on.
1phraseon the fenceUndecided between two positions.
“He is still on the fence about the whole thing.”