pH meter is a device used for potentiometric pH measurements.
In potentiometric methods you measure potential difference between known reference electrode and the measuring pH electrode. The latter depends on the activities of ions you want to measure. This dependence is described by the Nernst equation, thus once the potential has been measured you can calculate the activity.
First commercially available pH meter - Model G - was constructed in 1936 by Arnold Beckman. Since then pH meters became one of the most popular and most important measuring devices used by chemists and non-chemists in labs, field and industrial environment.
pH meter is nothing else but precise voltmeter, connected to the pH electrode, and scaled in such a way that it displays not the measured potential, but ready pH value.