What kind of goldfish? Some have naturally "popped out" eyes.
However, if the eye is rapidly getting bigger, then it is likely a problem.
Much more information is needed to help you:
Tank size, history (how long set up, etc)
Age of fish, how long have you had it?
Water parameters, pwc history
Tank mates?
A picture of the eye?
This is likely one of 3 things:
1. An injury - in which case a bit of salt & pristine water conditions are called for.
2. A bacterial infection (aka pop-eye) - anti-biotic food is needed (suggests something with Kanamycin - eg. Medi-Gold) together with pristine water conditions. Severe systemic infection (ie dropsy) may also give pop-eye. Check for raised scales ... treating dropsy is another subject altogether.
3. A parasite - eye or brain flukes (sometimes you will see white flecks inside the eye - the flukes themselves.) Praziquental containing food is suggested.