They aren't recommended for newbies or casual users, because they are difficult to set up securely and difficult to use safely, and there are a lot more things that can go wrong than using a software or hardware wallet. If you know what you are doing, though, then they are one of the safest methods for long term bitcoin storage.

You would assign heads the value of "1" and tails the value of "0" (or vice versa). Flipping the coin 11 times will give you an 11 digit number in binary. Convert that to base 10 and you get a number between 0 and 2047, which will correspond to a word from the BIP39 wordlist which contains 2048 words.