Science is less about certainty than it is about measuring the degree of uncertainty. For example, you never prove a hypothesis using the scientific method. Similarly, there is always a degree of uncertainty in the numbers used in calculations. The level of uncertainty is expressed in the number of significant figures used to report any measured value. For example, if you measure a volume with a graduated cylinder you may be certain the measurement is reported to the nearest milliliter. If you eyeball the amount of water in a drinking glass, you'll be lucky to get within a hundred milliliters of the actual value. So, how do you know how many significant figures to use? How does this relate to accuracy and precision? These are things you need to know.