It isn't a secret number but it's a 256-bit SHA256 output represented as a 64 char hash digest, in other words, it's any 64 char hash value that starts with the specified quantity of zeros, for example, if the bitcoin algorithm specifies that it must start with eight zeros then a sha256 hash digest of 00000000a16506ee98e8de04e91638d614546a116580c7d81d5807616cba8096 will mine the block, any hash digest that starts with eight or more zeros will mine the block. There are multiple correct answers, first, to guess the correct answer mines the block, gets the reward.