When you have uncertainty, that means you're not really sure. If you missed a lot of classes and didn't study much, you may have uncertainty about whether you passed your big math test.
The noun uncertainty describes a state of doubt. You might view with uncertainty a claim that a $100 investment will return $500 by next month. Uncertainty can also involve being dependent on chance. Your uncle might have uncertainty that he'll have a job next year because his company might be closing one of its branch offices.