For most of us when we think about how something looks, for example when I think about how an apple looks I have a conscious experience of what scientists and philosophers call Qualia of the apple, a fancy word to say that I have a vague conscious experience of seeing the apple in my mind’s eye, yes I actually see it!
This is in no way the same as actually seeing an apple I hold in my hand, but I do have moments or glimpses that are kind of graphic and colourful and look just like the apple. Although this is effortful and I cannot maintain it for any long period of time, it simply comes and goes as I put effort into holding that mental image in my mind.
Now aphantasia is the complete lack of this experience or being blind in the mind. The inability to visualise things – so not seeing any apple when I think about the apple.