Your body’s lymphatic system is, like the internet, a series of tubes. They’re sort of built like your veins but under even lower pressure so they rely on the movement of your muscles to squeeze the lymphatic fluid inside them around.

Lymphatic fluid carries a lot of weird immune shit around and is very important, but it’s also a prime wellness pseudoscience target. If moving lymph around was the primary benefit of exercise then laying down in a pressure chamber and turning it up and down would do the same thing.

Exercise does move lymph around but much more importantly it activates a zillion beneficial metabolic pathways in pretty much every other tissue in your body. It’s essentially the stimulus that tells your whole body “hey you need to be able to function so go ahead and do that.” It’s also incredible for ADHD, I sleep like boiled dog shit if I don’t exercise and it makes brain fog infinitely worse.

Lymph is the non-blood liquid that surrounds and is sometimes in some of your tissues. It’s related to the circulatory system through osmosis and similar processes but it moves around a lot better if you move your body. It’s sampled by your immune system, particularly in lymph nodes, which also act as a reservoir for various WBCs.

If you have a very sedentary lifestyle then your lymph doesn’t move around very quickly and so the processes it’s involved in are less efficient. This can mean a less effective immune system, fewer nutrients and more waste products near various tissues, and self-sustaining processes like localized inflammation cycles that stick around longer than might be ideal. This is one of the reasons that doctors usually recommend movement after injury, not just being sedentary and resting (the usual other reason being preventing scar tissue). Many of your body’s processes work better if lymph is circulating due to moving your body.

I mentioned it in this case because brain fog can be caused by many things and some of them are related to the processes the lpjatic system is responsible for.

Others have given you info we know. The funny answer, and the reason you couldn’t get much clarity, is that science is generally not very sure what lymph is, it’s a fluid of stuff, including lots of immune stuff, that helps with basically everything for reasons beyond our understanding. And moving around physically moves it better.

Biomedical science basically ends there