Imagine you recently visited a pine forest. If you later saw a picture of a pine tree, would you know whether it was a tree you had encountered before? Most of us wouldn't, since all trees of a given species look basically the same. This is a common way to describe the experience of prosopagnosia. People with this condition generally can't tell one face from another; for them, walking through a crowd of people is like walking through a forest.
Prosopagnosia happens on a spectrum, with some people only having trouble differentiating between the faces of strangers and others having a complete inability to know whether what they see is a face or an inanimate object. Some people It has been tricky for researchers to pin down exactly what portion of the population experiences face blindness, but it's estimated at 2 percent, or roughly millions of people. Many of these people may not even know they have it.
Related: Are You A Super-Recognizer?