To sit on the fence means to remain neutral, to refuse to take sides in a dispute. It is often used in a derogatory way about someone who lacks the courage to decide.

For example:

The councilman is afraid he’ll lose votes if he takes sides on the zoning issue, but he can’t sit on the fence forever.

I know everyone reading this will go for a 1-1 draw, but I don’t like to sit on the fence for big games like this, and neither should you.

The obvious prediction to go for is a draw – that’s what Kellie will go for, I’m sure, and I will sit on the fence too.

They need to be condemned, and there’s no place to sit on the fence about this: You’re either condemning it or condoning it.

Tradução: ficar em cima do muro, evitar tomar partido num conflito, discussão, briga etc.