There are two common causes

a) one or more of your spark plugs is weak or not working, causing unburnt fuel to exit through the exhaust. In such cases you would see noticeable loss of power and high fuel consumption.

b) Your O2 sensor is faulty, causing the air-fuel mixture to run rich (the mixture is more saturated with fuel than normal). Again unburnt fuel will exit through the exhaust. You will also observe signficantly higher fuel consumption in this case.

Other rarer causes include faulty ECU and faulty injector, among others.

In any case, you will need to get it fixed ASAP. Higher fuel consumption nothwithstanding, your catalytic converter will die soon if high amount of unburnt fuel continue to filter throught it.