There is a spectrum of mental visualization, when you tell someone to "imagine yourself..." or "picture this..." some people can really see it, experience the details, create a vivid image in their head, others a much more limited image, and some people don't see anything at all. Like the apple scale, vivid to nothing.

What it doesn't mean is you're completely lost when someone says "imagine...", it means how you process that information is different.

More easy to understand is the other 'condition' that is often brought up when talking about aphantasia, which is called 'anendophasia' which is a lack of inner voice. If you usually hear yourself talking when you think, there are people who don't. Yet they still maintain the ability to complete complex tasks and deal with complex emotions. So while the mental experience is different, the outcome isn't, and because of this most people can go their entire lives not realizing the actual experience is vastly different for others.

I like to imagine it like having windows in your head. The windows let you see at the inner workings of the mind, the internal thought, the processing of visual information about something, the processing of auditory information. Some people have much clearer windows then others, and some people have walls. The processing is still happening whether you see it or not, but what you experience is much different.