What is the document in relation to?
Jail does happen.
But the other party is well on his/her way towards getting a cost order against you. Basically, you will be directed to pay their court costs / legal fees.
If you refuse the debt can go to debt recovery, from there (I Believe - and I'm not 100% sure) but I believe you can find that you won't be able to do stuff like register a car / re-new your driver's licence.
You went to court, you had your opportunity to make your case. That is what the legal system is about - and impartial third party (the magistrate) made a decision. Severe penalties can be applied and enforced, I'd suggest you sign the piece of paper.
The other thing that can happen, depending on the nature of the thing you're being asked to sign is the court can sign on your behalf so the court order can take effect. So what will you have won?