In classical physics, a strict distinction is made between particles and wave phenomena. A central point of quantum physics is that this separation is not allowed in the microcosm. Particles behave like waves and waves like particles. It depends on the problem under consideration whether a description as a particle or as a wave is more suitable. This is called wave-particle dualism.
In a Mach-Zehnder interferometer, a light beam is first divided into two components by a beam splitter and then reunited at a second beam splitter. Caused by the optical path difference of the two partial beams, two complementary interference patterns can be observed on two screens behind the second beam splitter.
A Mach-Zehnder interferometer is very useful to illustrate quantum mechanical "which-way" problems. If you place a polarizer in each arm of the interferometer and its polarization planes are rotated 90° against each other, the interference pattern disappears. Of course, this observation can be completely explained by classical electrodynamics - but one can choose a quantum mechanical description if one reduces the light beam in the interferometer to single photons (or only a single photon). By inserting the crossed polarizers into the structure, the two possible light paths are made distinguishable - we get a "which path" information. Therefore the interference pattern disappears.
If a third polarizer is added between the second beam splitter and the screen, the so-called "eraser", which is oriented by 45° compared to the other two, all photons reaching the screen have the same polarization again. Since the path information is now lost again, i.e. "erased", the interference pattern will appear again.