When you send the photons through the double-slited wall, they form a diffraction pattern on the other side, which is a wave-like phenomenon. It makes no sense to think of the photons as "particles" anymore, since they would need to cross both slits simoultaneously in order to create the diffraction pattern.
So a way to find out what's happening is to place photodetectors in the slits. But what happens when you place the photodetectors is that these absorb the photons which hit them, not allowing them to cross the wall. This means that we are able to find where the photon was going to cross, but we didn't let it cross the wall at all. The rest of the photons will cross the wall across the other slit, but they will not form the expected diffraction pattern.
But if we remove the photodetectors and send the photons trough the wall, even in tiny packets small enough to consider that we are sending them one by one, we get the diffraction pattern. This would mean a single photon is crossing both slits simoultaneously, which is definitely a wave property and not a particle one.
I reccomend Feynman's lecture on the subject or the first chapter of Quantum Mechanics by Claude Cohen-Tannoudji.