Brain fog is cognitive impairment, where you have trouble concentrating, following the gist of something, or recalling things. Oftentimes short term memory is most affected.
For me it was a very sudden realization, back in 2015, that my brain simply wasn't working like it used to. I'd watch TV shows and then forget what I'd just watched. I began to panic when I understood how bad things had gotten. Basically I'd tune out people when they were talking, be unable to make sense of certain things, and have real issues with basic recall. This has been a permanent problem since then. It's a daily struggle. Things that have happened in recent days, weeks or months are fuzzy in my mind - they don't register as clear memories. I have trouble with things like the chronological order of things, absorbing written information, and registering time and distance. I feel like I'm in a permanent daydream state. Basically my mind is always elsewhere.
It's most apparent to me when someone is telling me a story, or if I'm trying to follow a show or movie. I'm more or less aware of what's going on, but I'll struggle to tie it all together. My brain will drop things all throughout. So I'll miss crucical elements of the story or plot. TV and videos are like moving wallpaper, rather than something I can engage with. Everything feels like it's happening too fast for my slowed brain to absorb. If I don't make a conscious effort to focus hard on a very specific thing, it'll be lost on me. My mind feels like it's constantly swirling, or like static from a channel that doesn't come in. I can't think clearly or plan ahead with that kind of smooth thinking that I used to have and take for granted, so I often feel overwhelmed from everyday situations. More specifically, ones that don't go quite as they should (and require me to think on my feet).
It's its own kind of hell really, and difficult to explain to people. My world always feels fractured and blurry, and it's not something most can people can identify with. I've gotten used to it myself, but I'll never fully get over it.
There's no medical reason behind it that they can identify, and no meds I can take for it specifically. If you can't establish a cause, you can't really hope to get a cure. I assume I'll have it the rest of my life.