Significant figures
Definition
The digits in a measurement that actually mean something — every digit you are sure about, plus one estimated digit. They stop you from claiming more precision than your tools allow.
Technical definition
The digits of a measured or computed value that carry meaningful precision: all nonzero digits, zeros between them, and trailing zeros in a decimal. Results of calculations are rounded so they never show more significant figures than the least precise input.
Example
A ruler marked in millimeters lets you measure 12.3 cm (three significant figures — the 3 is estimated). Reporting the length as 12.3456 cm would be dishonest precision; your tool cannot support it.
More digits does not mean more accuracy. A calculator happily prints ten digits, but if your inputs had three significant figures, seven of those digits are fiction. Also, exact counted numbers (like 4 tires) have effectively infinite significant figures and never limit precision.