insoluble = speeds up digestion, laxative, gets the bowels moving, typically nonfermentable (e.g. 0 calorie), typically found in green leafy shit. typically more stealthy but traps sulfates and although you fart less, whenever you fart the smell will be absolutely vicious
soluble = slows down bowel movements, basically turns into a big slimy glob in your intestines and helps sop up some cholesterol and slows digestion down for long lasting energy, some are fermentable though so they get broken down into short chain fatty acids in the intestine and thus have calories (though usually around 1-2kcal per g only). makes your shits huge and gives you lots of benign gas.
you need both to keep regular bowel movements. most sources from veggies are majority insoluble, most from grains and legumes are soluble. if you go lo-carb, try supplementing with psyllium husk though i like to have the occasional high fiber protein bar with resistant starch. otherwise eat a fuckton of legumes and spinach and you'll be fine.