The counter electrode is one of the three electrodes used in a classic 3-electrode setup of a potentiostat. This setup is used to study reactions happening at the working electrode with a known potential applied.
The potential is applied between the reference and the working electrode. To protect the reference electrode from potential shifts, the current flows between the counter and working electrode.
The counter electrode is often an inert material with a big surface area. Platinum wires or meshes are popular counter electrodes. Also graphite rods due to their large surface area and low prices are used under mild conditions.
More details are available in this article of the knowledge base.
This section gives a short overview of the three types of electrodes (working, reference, counter electrode) you will encounter while using a potentiostat. It is explained how these electrodes look like and what their task is.