When someone says they have 'brain fog' what does that mean?

"Brain fog": You'd have to ask the specific person who says it, to be sure. Many people who use this term say they feel confused, have a bad memory, go into a room and forget why they're there, can't make decisions, feel overwhelmed when posed with thinking tasks, etc. Some (not all) people using this term may have chronic fatigue syndrome, fibromyalgia, or are receiving chemotherapy ("chemo brain").