A vernier has two scales, an indicating scale and a data scale. These move past each other, usually on a slide.
When the measurement is taken, the zero point of the indicating scale will lie at the true datum of the measurement. This will usually be between two gradations of the data scale.
The vernier's indicating scale will have a series of gradations. One of these will align with a gradation on the datum. The number at the aligned gradation of the indicator scale will be the extra data of the measurement.
Verniers are common on sextants, machinsts' measuring tools (all sorts), and slide rules.