significant figures
Significant figures are the digits in a measured number that actually carry information, the ones you really know rather than made up. If your ruler reads to the millimetre, writing a length as 12.3 cm is honest, but writing 12.300000 cm would be pretending to a precision you do not have. Significant figures are how a number quietly tells you how well it was measured.
The counting rules: all non-zero digits count; zeros between non-zero digits count; leading zeros (as in 0.0042) do not count, they only place the decimal; trailing zeros after a decimal point do count. So 0.0042 has 2 significant figures and 1.230 has 4. When you multiply or divide, the answer keeps as many significant figures as the least precise input; when you add or subtract, you line up the decimal places. Round only at the very end.
The point is intellectual honesty: your answer cannot be more precise than your measurements. A calculator showing 3.6666667 for a result built from two-figure data is lying; you would report 3.7. Significant figures are a quick shorthand for uncertainty, though a full uncertainty statement (like 12.3 plus-or-minus 0.1 cm) is more informative.
A rectangle measured as 2.0 m by 3.14 m has area 6.28 m^2 on the calculator, but 2.0 has only two significant figures, so the honest area is 6.3 m^2.
The answer is no more precise than the roughest measurement.
A trailing zero can be ambiguous: '100 m' might be 1, 2 or 3 significant figures. Scientific notation removes the doubt: 1.00 times 10^2 m is clearly three.