The difficulty target is a threshold. The hash of a new block’s header must be less than or equal to this value. The lower the threshold, the harder it is to find a block (more leading zeros).
The Difficulty target changes dynamically every 2016 blocks. The goal is to produce new blocks every 10 minutes. If hash power is added to the network and blocks are found faster, the difficulty threshold decreases, thus making it harder to find new blocks.