This indicator refers to the difficulty encountered in mining the last block of a specific period. Essentially, it shows how hard it was for miners to find a new valid block on the blockchain. The mining difficulty is adjusted regularly and depends on two main factors: the competition among miners and the total amount of computational power being used on the network.

To understand this simply, imagine Bitcoin mining as a competition to solve very complex puzzles. Each block in the blockchain is like a unique puzzle that needs to be solved. The more miners there are trying to solve these puzzles (i.e., the greater the competition) and the more computational power they are using, the harder it becomes to solve the puzzle.

This regular adjustment of difficulty is important to maintain the network's stability and the regularity with which new blocks are found. Without this adjustment, if many miners join the network and start using powerful machines, blocks could be found too quickly. Conversely, if many miners leave the network, it could take a long time to find blocks. The difficulty adjustment helps to maintain a balance, ensuring that blocks are found at a steady pace, which is crucial for the maintenance and security of the blockchain network.