This refers to how easily a data block of transaction information can be mined successfully. A high difficulty level means that more computing power is needed to mine blocks. Of course, this means that the higher the difficulty level, the more secure the network is against attacks. For the purpose of mining, the Bitcoin blockchain has an additional field per block: The "Nonce" ("number used once") field contains a number that is unique in the respective context. Furthermore, there is a hash value that it has to start with a certain number of zeros. The number of zeros that the hash must start with is the so called "Difficulty". For example, for Difficulty 0000, the miner must find the nonce number that, when combined with the data in the block, results in a hash that starts with four zeros.
