Basically an SHA256 value for any given input is unpredictable so trying to find a result like this one (note the zeros at the start)
00000000916dd8861312f2cd94ac01310e15541e58df57d988f058eb3adcbf15
is not something that you can guess how to find quickly (it all depends upon the input string and how SHA256 mangles that data into a hash).
In Bitcoin's case the # of leading zeros is what is termed the "difficulty" and each miner creates a block header and then hashes this with a random number in order to find a hash with enough leading zeros (according to the current difficulty level).
Because there is no way to cheat to determine which random number will work you have to keep trying different numbers until you find one that does (thus the "proof of work").
HTH