Aphantasia is the inability to “see” or “hear” in your head. While it’s surprising to learn that people think differently (both for people with or without aphantasia), it has almost no consequence for daily life, except maybe for making it more difficult to remember past events.

The wiki gives a pretty good presentation of the phenomenon: https://en.wikipedia.org/wiki/Aphantasia

It only started to be studied recently; and the “visualizing a red apple” has become a meme on that topic.