What is a doubt or uncertainty in measurement?
Basically your uncertainty is the innaccuracy or your measurement. For instance if you had a yard ruler that was marked only in inches and the length of the object you were measuring lied somewhere between 12 and 13 inches; you could state that the objects length is 12 1/2 inches ± 1/2 inch. The ± 1/2 part is your uncertainty, it means the measurement could be either 1/2 inch longer or shorter than your stated measurement.