Measurements are never exact. There is always some uncertainty, usually due to limitations in the measuring instrument or in the human senses.
Accuracy is the closeness of a measurement to the actual value. Precision is the closeness of multiple measurements to each other. For example, a ruler that has more divisions will give you a more precise answer, but not necessarily a more accurate one.
Uncertainty is the amount that a particular measurement could be off by. It can be expressed as absolute uncertainty, like (20.4 ± 0.3) kg, or as relative uncertainty, like 20.4 kg ± 1.5%. Relative uncertainty can be calculated from absolute uncertainty with
`"RU" = ("experimental uncertainty")/("measured value") xx 100%`.
When adding or subtracting measurements, add up the absolute uncertainties. When doing anything else (multiplication, division, square root, etc.), add up the relative uncertainties.