Sonder is not a widely recognised word. 
John Koeing has coined the word as part of his book 'the dictionary of obscure sorrows' and it is defined as the "realisation that every random passerby is living a life just as complex and vivid as your own".

These images com from a photo-essay created as part of a project to visualise this immense idea. Photographing random people in the street in a state of total unawareness, Cameron created a the essay comprising it of several photos and accompanying text.