Aphantasia is a phenomenon in which people are unable to visualize imagery. While most people are able to conjure an image of a scene or face in their minds, people with aphantasia cannot. If you were to ask a person with aphantasia to imagine something, he or she could likely describe the object, explain the concept, and rattle off facts that they know about the object. But that person would not be able to experience any sort of mental image to accompany this knowledge. You can view the video of a woman describing her aphantasia below.
There is a website that is a network for people with aphantasia: https://aphantasia.com/