The 24 words are picked randomly from a list of 2048 possible words. So the 24 words are digits of a base 2048 number. That means there are 2.9642775e+79 possible combinations (in math it would be written as 2048^24), which is a 256bit key.
This is the encryption key to unlock the secret material that is your identity for your accounts. So while it is simple, it provides a large number of random bits and provides the highest level of encryption in use today.