Albert Einstein said that. I believe it may have been in reference to Schroedinger and his cat.
One of my pastimes is reading about quantum mechanics, particle physics and philosophy. Einstein believed in a set order of the universe, Schroedinger believed in the universe changing depending on observation. Nothing happens unless it is observed. The cat is both alive and dead, and neither dead nor alive, until observed.
Light is both particles and waves. It reacts differently depending on the observer. Light is a wave if you are looking for waves, but it is comprised of particles if that is what you are seeking to observe.
I still can’t wrap my head around that, but it has been proved experimentally.
One of my favorite books is out of print, “The Dancing WuLi Masters” by Gary Zukov. It explains quantum mechanics in layman’s language, without all the mind-boggling math at which I am barely sufficient.