Master the different rounding rules, from standard half-up rounding to bankers rounding, and learn about significant figures.
Rounding makes numbers simpler and easier to work with. The basic rule (often called "half-up") is: look at the digit to the right of your target place. If it is 5 or greater, round your target digit up. If it is 4 or less, leave it the same.
Sometimes specific rounding modes are required. "Ceiling" ALWAYS rounds up (towards positive infinity), while "Floor" ALWAYS rounds down (towards negative infinity). "Bankers Rounding" (half-to-even) is a method where exactly halves (like .5) are rounded to the nearest even number. This reduces statistical bias when adding many rounded numbers together.
In science, numbers represent measurements, and no measurement is perfectly precise. Significant figures are the meaningful digits in a number. Rounding to standard significant figures ensures that calculations do not imply a false sense of precision.