Bitcoin mining means distribution of timestamp server as peer to peer network. All the machine which are connected through peer to peer will continue authorising transactions and the process is known as Bitcoin Mining. 

For the Bitcoin timestamp network, a valid "proof-of-work" is found by incrementing a nonce until a value is found that gives the block's hash the required number of leading zero bits. Once the hashing has produced a valid result, the block cannot be changed without redoing the work. As later records or "blocks" are chained after it, the work to change the block would include redoing the work for each subsequent block.

To compensate for increasing hardware speed and varying interest in running nodes over time, the difficulty of finding a valid hash is adjusted roughly every 2 weeks. If blocks were generated too fast, the difficulty increases and more hashes are required to find a block and to generate new bitcoins