1) The SHA-256 hash of a block's header must be lower than or equal to the current target for the block to be accepted by the network. As mentioned in https://en.bitcoin.it/wiki/Target

or,

2) The hash of the block has to be less than the target. As mentioned by Andreas Antonopoulos here: https://youtu.be/h429LCTRmQw?t=1m28s

why not 9 min or 11? why it has to be 10?