The bitcoin miners have to secure the network and process every transaction. They can attain this by solving computational problem that allows them to chain together public blocks of transaction called block chain.
Bitcoin is not difficult but has become more competitive in nature. Because the SHA-256 hash of a block's header must be lower than or equal to the target for the block to be accepted by the network. Valid blocks have a hash below this target. Mining pools also have a pool-specific share difficulty to set a lower limit for shares.