It is, of course, forbidden to cook on Shabbos. On the other hand, we all want hot food on Shabbos. Actually, it is more than "want", we are actually making a strong statement that we believe in the Torah sh'B'al Peh and that Chazal have accurately transmitted it to us. All that from having cholent? You betcha! Every time you enjoy the radiance of the lights at your Friday night s'uda and eat a bite of tasty hot cholent, you are saying that HaShem gave us the Torah with all it's instructions at Har Sinai and we are faithfully adhering to that divine standard till today.
Chazal also built safeguards to prevent us from slipping out of the heavenly bliss of Shabbos that is mei'ein olam habah into ... well, the warmer place that isn't so friendly. One of the mainstays of that safeguard system is the friendly old blech; covering the flame with a metal cover that is not usually there. The blech accomplishes two things. First, it is not the normal way of cooking (that's why it has to be something that is not usually there); it makes the situation obvious that today is different. Second, it reminds you not to stir the coals.
What's that? You use a gas or electric range? You don't cook on open coals in your kitchen? Hmm... fascinating. Actually, because of that, many poskim (R' Fuerst, among them) recommend being stringent and to also cover the knobs. Many blechs (ours included) have a lip on the front that covers the knobs. B'di'avad, just covering the flame/burner works, but l'chatchila the knobs should also be covered.
Some (not poskim) want to be really clever and just cover the knobs since they don't have open coal fires on their ranges. They are wrong. Just covering the knobs is not good enough according to anyone (except, of course, non-poskim). However, there is an eitza given by the Rema; which is to plaster the knob into position so it can't be moved on Shabbos. The M'chaber disagrees, but we pasken like the Rema, so that works also.
What's that? You don't have a ready supply of plaster to freeze the knobs in place? Oh... and your wife will be less than happy with you plastering her range? The Chazon Ish has an interesting kula for you. (Yes -- kula. He had to have one, apparently.) The Chazon Ish says that the basis of plastering the knobs is to add another step before you could actually adjust the heat. Therefore removing the knobs, taping the knobs, or even putting a sign that says "Do not adjust! Shabbos!" (in a language you understand; lashon hakodesh doesn't work for this if you don't understand it) all work just as well as plaster. And your wife will be happier.
Note, please that this only works for sh'hi'ya (leaving the food on the heat), not chazara (returning the food to the heat); you'll need a blech for chazara. Also note that others argue on this kula of the Chazon Ish. Who and why? Listen up -->