Foods like cauliflower, broccoli, and so on, have relatively low glycemic indices. There are carbohydrates available, of course, but the sugars and starches in those vegetables are tangled up with indigestible fiber, cellulose, and so on. If you eat these foods, your stomach needs to grind on them for a while to get out the nutrients, which will slowly enter your bloodstream. In addition to providing valuable fiber and probably more vitamins than what's in a can of Coke, since the sugar enters your bloodstream slowly, only small amounts of insulin need to be produced to perform the digestion. See the page on health benefits of the caveman diet for more information about insulin level control.
Foods can fall anywhere on a range, so the terms "high glycemic" or "low glycemic" are both relative terms. The actual numbers usually represent how quickly the carbohydrates can be absorbed compared to a food with a very high index, like pure refined sugar. If pure refined sugar is 100 and you eat a food rated at 20, that means that it is 5 times more difficult for your body to absorb the carbohydrates in that food than to absorb the pure sugar.
So the basic idea is this: when you've got a choice among different foods, generally try to choose those with lower glycemic indices.
The advice above is pretty good, but it can even be improved slightly by taking into account not only how easy it is to absorb the carbohydrates, but also how many carbohydrates there are. Parsnips, for example, have a very high glycemic index, but contain almost no sugar, so if you eat a parsnip your blood sugar isn't going to change much. Sure, all the sugar in the parsnip is very soon in your blood, but there was almost no sugar to begin with, so it won't cause any insulin spikes.
The glycemic load is obtained by multiplying the glycemic index by the amount of carbohydrates that are actually available.
In any case, here are a couple of pointers to get you started:
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