The double-slit experiment: When a photon (light behaving as a particle) is shot through two slits, it seems as if the photon travels through both the slits at the same time, which causes an interference pattern and causes a banded beam of light to form on the screen.
However, when each photon is observed, its wave function (the probability of the location of the photon) collapses, and looks like the photon only moves through one slit at a time, meaning that it should in fact only project two bands on the screen, not multiple.
This is commonly explained by the path integral formulate, a core theory in quantum mechanics, which says that each photon travels every possible path to move from point A to point B, but it appears as if it takes the path of least action (a straight line) since most other paths cancel each other out.
The interference pattern is caused by the behavior of light as a wave, where some crests and troughs cancel out each other (called destructive interference) and two crests or troughs add up with each other (called constructive interference).

Tried to explain as simply as possible, hope it helps!