Epilepsy Mode is a feature in Project Plasma that reduces the flashing of certain animations (such as bullets) to make the game easier to play for people who are prone to seizures (such as people with epilepsy.) A lot of bullets have rather quick flashing animations within their sprites or just generally move fast which can make the game uncomfortable to play for people who suffer from seizures. An example of where this becomes a big problem is on the level 15 boss fight against Davagon.
Those purple bullets flicker slightly (hence why some of the bullets there are a slightly darker shade of purple than others) which isn't noticeable if there aren't that many of them on screen, but in cases like the above where every bullet that is being fired from the boss is like that then it becomes very noticeable and that's when people start getting uncomfortable during gameplay.
Other cases where epilepsy can become a problem include some swarms on some levels, the boss fight with Crystalline, and some parts of the boss fight with Dynast. However the boss fight with Davagon is probably the worst when it comes to this.