The experiment is easy to do, but difficult to understand. It consists of setting up a light source on one side, facing a barrier (such as a piece of cardboard) that has two slits in it to permit some light to pass through to a distant wall on the other side. If both slits are open, then one pattern is observed on the distant wall. But if one slit is covered, then a completely different pattern is then seen on the distant wall.
This simple experiment demonstrates the wave-particle duality of light. Light behaves as a wave when both slits are open, such that the wave simultaneously passes through both slits and creates an interference pattern on the distant wall.
The experiment can also be done with particles, such that a series of particles are directed at a grid, or other similar device, while observing the pattern on the other side.