The wave particle duality of light means that light exhibits properties of classical waves in some cases and properties of classical particles in others. The fringing effect that results from firing single photons through a double slit is the most famous demonstration of this duality.
If light were only composed of particles, firing single photons through a double slit would be expected to yield an imprint of the two slits on a receiving screen behind the slit. Instead, the light causes fringes, reminiscent of the constructive and destructive interference patterns of interacting waves. Rather than passing through one slit or the other, the light particles pass through both slits at the same time. When passing through the slits, a photon is at the first slit to a certain extent and at the second slit to a certain extent. It is impossible to say for certain which of the two slits the photon passes through, a truism called the Heisenberg uncertainty principle that was postulated by one of the founding fathers of quantum physics, Werner Heisenberg.
Another contributor to the dual nature of light was Albert Einstein, who received the Nobel prize in physics for demonstrating that light has definite particle properties when he explained the thermoelectric effect in terms of photon-matter interaction.