It's when, due to high G forces, the blood is pulled to your feet meaning less will manage to get to your brain. This will affect you in several stages: on the first one (greyout) everything you see becomes grey/black and white and your sight loses "definition"; second one (tunnel vision) you lose peripheral vision meaning you'll only see straight forward; third stage (blackout) you just stop seeing; on the last stage you pass out, also called G LOC (gravity induced loss of consciousness).
This might seem dangerous but you would need to experience G LOC for several minutes to be harmed while on roller coasters you may only grey or blackout for a few seconds, so it's perfectly fine.