When making some kind of scientific measurement, it is necessary to first check your measuring instrument for 'zero error'. The zero error is the reading displayed when you know the true reading should be exactly zero.
For example, using a set of vernier calipers, the zero error is the reading that shows when the calipers are fully closed.
Zero error will often be present even on an otherwise accurate instrument; for example, a well-used pair of vernier calipers may have slightly worn jaws. This doesn't mean the instrument is no longer useful. As long as you check for zero error, you can then use it to correct your readings.
For example, if you record a zero error of 0.1mm, you know you must then subtract 0.1mm from all of your results.