system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is actually going on when I can hear myself think or see mental images?
gpt
It's disappointing to see some bad answers to such a good question. 

Your brain has many different functions. Two of these are seeing and language processing. When you see, you use a part of your brain devoted to that function. Same thing happens with language. 

Here's the good part: you can use these parts of your brain even without the external "stimuli" or rather, you can imagine an image by simply drawing on past memories instead of needing to look at something. Also, you don't need to actually hear anything to be able to "hear" words in your head. The visual and auditory aspects of the brain can just draw from memory. This is useful because synthesizing new ideas from past experiences is vital to being able to learn and adapt to ones environment. 

Tl;dr - what is happening? Your brain is synthesizing or creating visual and auditory things from your memory. The same connections in the brain are being used; just without the external stimuli.