Example: a measurement of 5.07 g ± 0.02 g means that the experimenter is confident that the actual value for the quantity being measured lies between 5.05 g and 5.09 g. The uncertainty is the experimenter's best estimate of how far an experimental quantity might be from the "true value.
Links
[1] https://hootsgo.org/?q=node/6455
[2] https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/introduction-to-chemistry-1/measurement-uncertainty-30/accuracy-precision-and-error-190-3706/