Precision, accuracy and tolerance
Precision is how closely repeated readings agree. Accuracy is how close they are to the truth. Tolerance is the allowed band.
A worked example
A rod specified at 6.25 cm ± 0.03 cm passes anywhere from 6.22 cm to 6.28 cm, a range of 0.06 cm.
The mistake people actually make
Readings can be precise and still wrong if the instrument needs calibrating. A ruler reading carries ± half its smallest division.